Cosmo and Wanda (seasons 9 and 10)

Cosmo and Wanda are two of the six main characters in The Fairly Oddparents and are Timmy Turner's fairies. They've been flanderized since season 6 until the show's cancellation.

Why They Suck
NOTE: This will only focus on how badly they were butchered since season 6 until the show's cancellation.
 * 1) Cosmo 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
 * 2) Wanda was also originally portrayed as being dimmer than Cosmo, believe it or not, then at some point, they shared an equal amount of intelligence.
 * 3) Since season 6, Cosmo became simply the Ditz as opposed to being both that and a cloudduckoolander.
 * 4) Wanda became simply the wiser one who chastised Timmy and Cosmo whenever they did something wrong, but then all her jokes became nagging.
 * 5) Speaking of nags, Wanda was one to the point where the Barbarian Hero episode had her become the whip cracking Nagules.