Lisa Simpson is the oldest daughter in the Simpsons family; older than Maggie, but still younger than Bart. She unfortunately received a lot of flanderization.
Why She Sucks Now
Lisa has gone all over the place, she was made the smartest and "best-behaved" character just in contrast to Bart.
She is also very political. This is very noticeable in the 2007 movie where she nagged the town about the lake being polluted, and in "Lisa vs. Malibu Stacy" where she gets offended over a simple talking doll.
Her intelligence was dialed straight up to 11. Outclassing her peers very badly.
She is a bigot. She has had some moments of being intolerant towards others opinions like how she calls Marge dumb for believing in angels, and she holds her vegetarian beliefs against anyone who is not a vegetarian.
Lisa became a hardcore activist and almost an extremist, her sardonic view became a Holier Than Thou superiority complex.
Her nerdy interests became a patronizing enthusiasm for the most pretentious of vocations.
Somehow, Lisa knows obscure facts that no kid her age, not even a smart one, would know or care about, such as when Texas was admitted to the Union, that Monaco has a prince, or what Occam's razor is, yet somehow she doesn't know where babies come from.
Simultaneously, Lisa became very uncool and lost most of the friends she had in the earlier seasons.
She is a huge killjoy in a lot of episodes
She is extremely egotistical and competitive.
In almost every single episode that involves her being in a conflict with something, she's always portrayed like she's in the right of the conflict, always comes out on top, and they are always at a desperate attempt to make you root for Lisa. Like in any episode where Bart is superior to Lisa at something, and the episode tries to paint Lisa as sympathetic despite that she's just being jealous of Bart, which is a petty reason to make her sympathetic.
She is shown to be willing to abandon her family in pursuit of glory. In "Elementary School Musical", she ran away to pursue a career in performing arts.
She's always butting into people's business and trying to force her opinions onto people when they don't ask for them or want them, basically making her as Ned Flanders calls her "Springfield's answer to a question no one asked".
She is extremely hypocritical, or self-serving in a lot of her causes. One infamous case being her motives for joining the school football team.
"You shouldn't be part of a protest if you don't care about the cause."
Lisa was still a far more likable and funny character due to the fact that she is still a Hollywood Walk of Fame character like Ms. Piggy, even in the old and new seasons.
She is likeable in some episodes post golden age like "Mother and Child Reunion", "The Way of the Dog", etc.
Similar to Homer and Bart, there are still episodes where she is at her best.
In The Man Who Grew Too Much, it's shown that she's for GMO, something that SJWs would attack, meaning that she's not always alt-left and she can give new science a chance, despite her flanderization.