Flanderization

Flanderization is a trope that refers to the act of taking a single trait from a character and exaggerating and amplifying it more and more over time until it completely consumes the character. Most always, the trait/action becomes completely outlandish and it becomes their defining characteristic. Sitcoms and other comedy shows are usually the ones to suffer this, due to the writers doing anything they can to get a cheap laugh from the audience, even if it means sacrificing a character's personality. Either there are out of character personalities that are counted as against the characters we have known in love, or there are personalities that will make you have no choice but to lose all of your respect you would have had for all the characters.

Although, Flanderization doesn't always make the characters turn out awful. Some Flanderization can make them funnier and likable. It depends on how the character is flanderized. This good side of Flanderization is most noticeable in Thor from the MCU, who became a less serious and more badass hero.

The Simpsons, being the series which spawned this trope's very name, has this almost across the board. Over time, the series has, in many ways, become a symbol of this trope in its purest form.

In the topic of Flanderization, the American animated sitcom King of the Hill is a notable example of a series that used Flanderization. This series is known for giving flatter characters new aspects, then focusing on them, thus creating the sense of Flanderization at the same time as they get Character Development.

Notable victims of Flanderization

Ned Flanders (The Simpsons; the trope namer, went from merely enjoying going to church to being obsessively religious and overly nice, and then later an obnoxiously intolerant bigot who will lash out at anyone that doesn't support Christianity)
Moe Szyslak (The Simpsons; became the show's punching bag, even before he was the show's punching bag, he was the main punching bag whenever he gets prank called by Bart, other things else, either done in a funny way or not so funny way)
Homer Simpson (The Simpsons; became a selfish and sometimes crazy moron.)
Marge Simpson (The Simpsons; turned into an overprotective mother who's no fun at all, what is completely against her character. This happens in the episodes where the episode is disliked by fans, considered boring, or is just plain bad)
Bart Simpson (The Simpsons; turned into a borderline sociopath, though not as bad as Peter Griffin)
Lisa Simpson (The Simpsons; became a know-it-all Social Justice Warrior, which is far too against her character)
Maggie Simpson (The Simpsons; became a flawless Mary Sue, despite the fact that she is likable and cute)
Garfield (The Garfield Show; became an unlikable slob)
Jon Arbuckle (The Garfield Show; became an idiot)
Nermal (The Garfield Show; became a jerk)
Harry Potter (Harry Potter novels 5-7; became an unlikable jerk)
Peter Griffin (Modern) (Family Guy; Season 7-present; became an insane, rude, hypocritical, selfish, psychopathic, and an egotistical person)
Lois Griffin (Season 7-present) (Family Guy; Season 7-present; became a sex-crazed, abusive short-tempered wife)
Meg Griffin (Family Guy; Seasons 6-14; became a punching bag up until Season 14)
Chris Griffin (Family Guy; Season 7-present; became a character going in full idiot mode in the worst way possible)
Brian Griffin (Family Guy Seasons 7-present) (Family Guy; Season 7-present; his entire personality now amounts to being a "liberal", and became a complete jerk and got even worse when he came back)
Glenn Quagmire (Family Guy; Season 7-present; became not just a pervert, but an implied child molester)
Stewie Griffin (Family Guy; went from a toddler with the mind of an evil genius to a toddler with the mind of a stereotypical gay guy)
Ed (Ed, Edd n Eddy; Seasons 5-6; became a too-dumb-to-live idiot, although he is still likable)
Edd (Ed, Edd n Eddy; Seasons 5-6; became a constantly complaining goody-two-shoes)
Eddy (Ed, Edd n Eddy; Seasons 5-6; became an unlikable violent maniac)
Jimmy (Ed, Edd n Eddy; Seasons 2-6; became a vengeful camp gay crybaby)
Jonny 2x4 (Ed, Edd n Eddy; Seasons 2-6; became an annoying loudmouth goofball and saboteur who can't act or function without Plank)
Spongebob Squarepants (Spongebob Squarepants; Seasons 6-8; became an obnoxiously happy moronic idiot and a crybaby in some episodes)
Patrick Star (Spongebob Squarepants; Seasons 1b-5; 6-8; he suffered two flanderizations; he started out as a kind of dumb but fairly intelligent, but as Season 1 progressed, he became more stupid and immature and eventually his stupidity became his main, defining trait (This was a good Flanderization that made him funnier and more memorable); then, in Seasons 6-8, he had second Flanderization (which wasn't a good one). He essentially became brain dead, with his actions becoming dangerous at times. He also became a selfish jerk. True, Patrick had his jerky and antagonistic moments in earlier seasons, but these moments pretty much became a major part of him in these seasons. Then, in Season 9 onwards, he finally got better after years of complaints from fans and critics)
Mr. Krabs (Seasons 6-Present) (Spongebob Squarepants; Seasons 6-8; turned into an even bigger black-hearted and self-serving greedy monster who, in one episode, terrorized Plankton to the point where he attempted suicide (yes, seriously) and Mr. Krabs just brushed it off like it was nothing; his love for money also became over-exaggerated)
Timmy Turner (The Fairly OddParents; was portrayed as essentially a good kid in the first few seasons, who occasionally gave into his selfishness. However, he was still very likable. However, by about the fourth or fifth season, he became somewhat of a jerk along with all the other denizens of this hellish parody of sitcom America, including his Fairy Godparents who were summoned to help him the first place. Timmy has returned to most of his original affable characterization by the sixth season onward but it still shows up every now and again, before relapsing back into it in the nineth and tenth season upon the introduction of Chloe Carmichael)
Cosmo (The Fairly OddParents; was originally simply The Ditz and a bit of a Cloudcuckoolander, sometimes dim but still relatively normal, and perhaps strangest of all, he and Wanda seemed to be on an equal level of airheaded and careless intelligence ("We're two halves of a whole idiot!") Starting around Season 2, he started to become increasingly dumber with each season. Now he's just The Ditz. Just look back at his character in the original Oh Yeah! Cartoons pilot — he was actually a suave and sophisticated guy, barely resembling the character he became. Also, his relationship with Wanda in Seasons 4 and 5 was the foil for a lot of marriage jokes; in a couple of episodes, he claimed to be trapped in the marriage to Wanda and that she had tricked him into it. This characteristic was toned down in Season 6, when the birth of Poof turned them into loving parents)
Wanda (The Fairly OddParents; was originally portrayed as being dimmer than Cosmo, believe it or not, then at around Season 1 they shared an equal amount of intelligence. From Season 2 onwards, Wanda became simply the wiser one who chastised Timmy and Cosmo whenever they did something wrong. Then, all her jokes tend to be that she's a total nag, to the point where the Barbarian Hero episode had her become the whip-cracking Nagules. Her marriage to Cosmo also became one of the biggest sources of humor on the show, to the point where Cosmo made it appear that he was trapped in the marriage to Wanda; however, this was toned down in Season 6, when the birth of Poof brought them closer together and they are currently revamped as loving parents)
Mr. Turner (The Fairy OddParents) and Mrs. Turner (The Fairly OddParents; they go from busy and clueless to extremely negligent parents who have hinted at times that they don't truly love Timmy, and see him as a burden. This was actually the plot catalyst in an episode guest-starring the late Steve Irwin as 'The Bad Parent Hunter')
Hank Hill (King of the Hill; is known for preferring traditional things over trendy new ones. This starts to get overdone in the later seasons; case in point, "Get Your Freak Off". In this episode, Hank is more Amish than the Amish; he's almost medieval)
The Annoying Orange (The High Fructose Adventures of Annoying Orange; became more annoying than he was in the web series to the point where he is not even funny)
Eric Cartman (South Park; became an absolute villain rather than just a jerk; not to mention, his bigotry used to be only targeted toward Kyle)
Butters Stotch (South Park; became a very sweet boy, one of the few times a Flanderization made a character even more popular)
Randy Marsh (South Park; became a paranoid coward and a manchild)
Bloo (Foster's Home for Imaginary Friends; started out as being protective over and caring of Mac in the pilot whilst being only slightly mischievous and then became a "jerk with a heart of gold" type of character in the first season, but then eventually he lost the "heart of gold" part and just became a full-blown, straight-up, outright meanie whose jerkiness kept getting worse and worse as the show went on until he finally started leveling off around the end of the series)
Earl Devereaux (Cloudy with a Chance of Meatballs; became a rude and manipulative jerk in the TV show adaption)
Mater (Cars; became an idiot jerk in the second film)
Optimus Prime (Became a psychotic killing machine in Transformers 2: Revenge of the Fallen)
Ren Hoek (Ren and Stimpy: Adult Party Cartoon; same as Optimus Prime)
Son Goku (Dragon Ball GT and Super; became dumber and more man childish)
Chi-Chi (Dragon Ball Z, GT and Super; became an obnoxiously overbearing mother to her sons, Gohan and Goten, to the point where she attempted to take Gohan's infant daughter away from him as she couldn't stand the thought of her becoming a Z-Fighter when she grew up)
Broly (Dragon Ball Z: Broly - Second Coming; became a brainless lunatic who did nothing but scream "KAKAROT!" at the top of his lungs)
Miho Nosaka (Yu-Gi-Oh! 1998; became a selfish, hypocritical ditz who uncaringly strings Honda around when she clearly has no interest in him and just uses him for her own personal gain)
Sakura Haruno (Naruto; became a violent psychopath who attacks Naruto for no reason)
Sheen Estevez (Planet Sheen; Went from being a funny comic relief to an immature, childish and unfunny idiot)
Spike Bulldog (The Tom and Jerry Show; went from being a friend to Jerry to a mean, short-tempered jerk who hates both Tom and Jerry)
Tommy Oliver (Power Rangers; became an annoying ranger every season after season)
Manny the Mammoth (Ice Age; became a stereotypical over-caring father in Ice Age 4: Continental Drift and became dumber and dumber as films went on.)
Sid the Sloth (Ice Age; Became a stereotypical lazy bum in Ice Age 4: Continental Drift and became dumber and dumber as films went on.)
Max Russo (Seasons 3-4) is changed from a funny, laid-back, and yet, kind-hearted and dimwitted character with slight intelligence, to an annoying, aggravating, arrogant, jerky, and a too dumb to live-type character, who's only there for comedic purposes only, much like Patrick Star from Seasons 6-8 of SpongeBob SquarePants.
Red (Angry Birds Toons; Season 2-3; because an overprotective Gary-Stu angry bullylike jerk that likes beating the innocent pigs for fun and leaving them to starve. He is also very racist towards the pigs and is very mean to them for no real reason)
Chuck (Angry Birds Toons Seasons 2-3; Changed from a lovable, funny speed demon to an annoying idiotic jerk who cares more about looking cool than protecting the eggs, he also gets too much screentime in the newer seasons.0
Jim, Jake, And Jay, (The Blues From Angry Birds Toons; Seasons 2-3; became annoying troublemaking brats that also like to beat the pigs for fun.)
Matilda (Angry Birds Toons; Season 2-3; Became an OCD overprotective Neat Freak that wants everything to be perfect.)
Bomb (Angry Birds Toons; Seasons 2-3; Became a boring penguin with no personality and eats ice cream for no reason.)
The Pigs (Angry Birds Toons; Seasons 2-3; Became 1-dimensional villain characters who want to steal the eggs from the Birds for no real reason when they could’ve eaten something else, They also Became punching bags for the Birds.)
Book (Battle For BFDI; became an unlikable douchebag to Taco)
Greg Heffley (Diary of a Wimpy Kid: The Long Haul (film) became an idiot.
Frank Heffley (Diary of a Wimpy Kid: The Long Haul (film) is the same as Greg.
Susan Heffley (Diary of a Wimpy Kid: The Long Haul (film) became a selfish and egotistical mother who belittles her family and bashes modern technology. She can also be an idiot at times like Greg and Frank.
Sam Puckett (iCarly Seasons 4-5; Sam & Cat); Was flanderized twice. In the earlier seasons of iCarly, she started out as being an aggressive tomboy. During Seasons 4-5, however, she became somewhat less aggressive and more girly. Then, during Sam & Cat, she became a selfish jerk.
Cat Valentine (Sam & Cat) became even dumber than she was in Victorious to the point of being an absolute idiot.
Sam Wilson/Falcon (Marvel Comics; became out of character after becoming the "new" Captain America)
Mark Taylor (Home Improvement; Season 7; became a gothic weirdo.)
Agent 3 (Splatoon); Started out as the hero you play as that defeated Octarains reasonably because they stole the Great Zapfish, to a heartless racist jerk who attacked Agent 8 simply because He/She was an Octoling because Octolings used to be the bad guys in Splatoon, but this Octoling heard the Calamari Inkantation and became good and wouldn't attack Agent 3, yet, Agent 3 still wants to attack Agent 8 for the sake of being an "evil" Octoling. Thankfully, Agent 3 became good in Octo Expansion because he/she saved Agent 8 and Cap'n Cuttlefish from being blended alive and was nice to agent 8 and was self-aware that not all octarians are bad guys.)
Wonder Woman (2011); Became a selfish, cold-hearted and murderous bully.
Blossom, Bubbles, and Buttercup (The Powerpuff Girls (2016)); (Blossom became a neurotic geek, Bubbles became one of the girliest girls while still retaining some of her more psychotic episodes, and Buttercup became a bully).
Jerry (Gene Deitch Era); Became a flat-out sadistic jerk who enjoys torturing Tom for fun.
Eustace Bagge (Courage the Cowardly Dog in Ball of Revenge^TTSW); Became a murderous jerk who wants Courage dead because he gets all the attention from Muriel.
Buddy Thunderstruck (Get the Hock Out and Cannonballistic; became a despicable and immature driver who is more dumber and edgier than his usual self who usually does stupid things and often gets himself in trouble with other people in front of him and act like he doesn't care for the bad things that happen to them or even himself to the point of him never getting told off or punished for his mean-spirted and stupid actions, a great example is when he and Darnell give Mr. Weaselbrat back his hot tub, he literality does a cannonball in the hot tub which has no water, doesn't listen to Darnell, lands in the hot tub and broke all his bones like an immature idiot despite him being tough enough to land on the ground and crash though places without even breaking his bones when in and outside the Rabble Rouser).
Darnell Fetzervalve (Buddy Thunderstruck in Get the Hock Out and Cannonballistic; became the writer's cowardly Gray Stu pet and unlikable idiot who gets forced to do bad things that Buddy should've done earlier like urinating on things and firing Cannonball from his new jobs as a Zeil's Truck Shop employee and a co-announcer at the Greasepit Country Speedway for his and Buddy's actions).
Big Tex Arkana (Buddy Thunderstruck) in Get the Hock Out^TTSW) and Thunderstruck Rab Cab; became an even greedier and villainous Mr. Krabs wannabe who often does illegal things such as hiring two people (Buddy and Darnell) with no money just to scam and take people's items away despite them already paying for them in his pawnshop property and stealing other people's driving secrets (especially Buddy's), yet he never got his pawnshop sued or himself arrested for these crimes).
Artichoke (Buddy Thunderstruck; Post Get the Hock Out; became a harmful replacement of "Sheriff Cannonball" who rarely shows charm and only threatens to harm and say very edgy things to other people when he gets distracted or stole his motorcycle).
Muncie (Buddy Thunderstruck in Get the Hock Out; became an know-it-all Lori Loud clone who barley helps her depressed costumers about what happen earlier despite being the most decent person in Greasepit who would help other people including Buddy with other things).
Auntie Uncle (Buddy Thunderstruck in Get the Hock Out]; became an extremely braindead and spoiled party animal with zero redeemable qualities who only cares about losing Buddy's money to other people and treating him like a homeless and poor person and even sell all of Buddy's and even her own furniture to Big Tex just to "keep the business going" which could gave Patrick Star or Peter Griffin a run for their money).
Robby Burgles (Buddy Thunderstruck in Get the Hock Out; became a pointless stereotypical repo man assistant to Big Tex with stomach problems which caused him to be only there for bathroom humor).
Mama Possum (Buddy Thunderstruck in Post Get the Hock Out; became a dumb, whiny and annoying moron who often gets herself and her babies in danger, especially in Get the Hock Out when two thieves show up for her birdbath while trashing her front garden, she told them to take her birdbath instead of calling the cops on both of them).
Star Butterfly (Star vs. The Forces of Evil; Went from a curious fairy princess wanting to fight bad guys to a spoiled brat who doesn't care about anyone or anything except to be with Marco.)
Marco Diaz (Star vs. The Forces of Evil; Became an uninteresting Gary Stu.)
Daenerys Targaryen (Game of Thrones Season 8 Part 2; Became a poorly written mad tyrant and destroyer of the common.)
Pac-Man (Pac-Man and the Ghostly Adventures; went from a video game icon who turns out to be lovable to a glutton who focuses more on food rather than helping others.)

The Overall Reason Why This Trope Sucks

Basically, this trope serves as reverse character development, where the characters you either love or hate (gets worse or gets ruined and lose any likability in them).