Ma Vreedle and the Incurseans also got some accusations of being this. Derrick Wyatt contributed to create both, and both were introduced in Alien Force and Ultimate Alien as minor, mostly comedic villains. And now that Wyatt is in charge, suddenly the Incurseans are upgraded to Big Bad rank for an entire arc, making them the first major baddies in the whole franchise to not be Knights of Cerebus, while Ma Vreedle made Vilgax cry. To add insult to injury, Vilgax got Demoted to Extra with his first episode being essentially a filler in the Incursean story arc.
Midway through season 2, Drawn Together essentially became The Captain Hero Show. Practically every episode focused on him or featured him heavily, to the point where fans of other characters were sick to death of him. The writers admitted that they liked to give him stories, their justification being that the guy was The Ditz and had no morals or sense of restraint so they could make him do absolutely anything (an important factor in a show where 95% of the jokes are Cringe Comedy) and it would never be out of character for him.
Family Guy: Brian Griffin. During the shows run, Seth MacFarlane has admitted that his intentions were for Brian to be the show's most popular character, while Meg would be the least popular, but to his complete surprise, Stewie became the show's breakout star, while Brian became the show's scrappy. The crew seems often desperate to make Brian more popular, at one point even killing him off then bringing him back just three episodes later, which fans would've been happy to hear, until they realized that these Family Guy episodes took six months to produce. They started to give Brian more noticeable flaws in the later seasons to prevent further backlash.
Mrs. Crocker, Denzel Crocker's mother, has become a minor one. Most fans see her just a plain, boring, and obnoxious old lady and even then for some reason other than being related to Denzel, since Season 7 onwards, the writers think that, with Denzel and Timmy's dad, she is an entertaining and funny character that everyone wants to watch.
Cubert from Futurama turned into a parody. He was actually created to be one of these, apparently in direct reference to the former Trope Namer, and he was originally intended to be a sort of The Ace character, except he would still be hated and mistreated by the other characters. Really only the last part became true. In his debut episode, the rest of the cast takes an instant dislike to him and manage to get in a few good insults. It helps that the writers were more in-tune than others in their profession. Originally Cubert was supposed to be the character who lampshaded all the ludicrous aspects of the series, which he does in his debut episode. However, the writers realized how terribly annoying this was and transformed him into a standard Bratty Half-PintTV Genius and only used him in a couple of episodes there after. He is also downgraded from a genius to an insufferable brat who thinks he's smarter than he is, while still acting like an actual twelve-year old.
There were (and to a degree, still are) so many restrictions imposed on Disney writers when dealing with Mickey Mouse, that any other character in any given Mickey cartoon practically got a carte blanche to steal the spotlight. The strange irony is that this happened because Mickey was the Creator's Pet of Mr. Disney, not the other characters.
Donny from The Powerpuff Girls (2016). He is abhorred by the older Powerpuff Girls fanbase mostly due to the misinterpretation that he was meant to be a metaphor for trans people. His debut episode is, however, one of the most infamous episodes in the reboot due to the fact, if interpreted as a transgender metaphor, its message is negative at best. Donny is also disliked for being seen as annoying. Unfortunately, the creators didn't notice this backlash in time. They had expected him to be a hit amongst the fans, likely because he's a unicorn (which didn't happen because the show was Screwed by the Network and not very popular amongst kids), and as a result he reappears in several episodes, including season 2 episodes (which were made right after season 1 and thus before the backlash). He's even featured on a DVD for the series named "The Last Donnycorn" (which is also the name of an episode).
The concept of the Creator's Pet was parodied brilliantly in Robot Chicken. It was done in a sketch featuring the producers of Star Trek: The Next Generation discussing how to make the fans love Wesley Crusher, after the fandom has paid for a billboard threatening to ass rape Wil Wheaton to death. So they decide to include a character so annoying, it would make Wesley look better by comparison. When one writer suggests they try to make Wesley a better character, he gets thrown out of the room. The resulting episode has an annoying Great Gazoo rip-off called "Snirkles", who proceeds to bother Wesley by playing a "space banjo song" and then disappearing. The fans then change the billboard to say "Kill Wesley. Keep Snirkles." Wil Wheaton, after seeing the sketch, tweeted that he would've loved to voice Wesley if they asked him to.
The Simpsons: Though it's not really a single character, "As Himself" is something of a taboo phrase in Simpsons fandom. Most of the time, especially after Season 10, any guest star to play themselves mostly shows up with little reason, acts cool and popular, and isn't really given anything funny to say. Hence, the star comes off as less there to service the plot, and more as a means of mooching off their popularity. Infamous examples include the stage magicians in "The Great Simpsina" and Lady Gaga in "Lisa Goes Gaga". The latter in particular's spotlight episode was massively-promoted and played very heavily on her public image without saying anything particularly new or interesting about it. Many contrasted her performance to that of Michael Jackson, who guest-starred as a fat white bricklayer living in a mental ward and received a good amount of acclaim for his part.
Tiny Toon Adventures: Somebody at Warner Bros. really liked the character Elmyra Duff since she took up a lot of screen time to the point where they tried to get a spinoff series off the ground with two pilot episodes about her and her family. That wasn't enough, though. She went on to make cameo appearances, as well as a guest-star appearance, in the show's successor series, Animaniacs. Even that wasn't enough. The spinoff series of Animaniacs, Pinky and the Brain, received a unneeded revamp after its third season, where the duo have become Elmyra's pets.
Owen in the first two seasons. The writers and voice actors of the show often wrote glowingly of him (Christian Potenza is quoted as having a man-crush on him), yet he is considered a Base-Breaking Character due to his constant Toilet Humor, Character Shilling (such as Gwen calling him sane even when he does exceedingly odd things, and the rest of the cast talking about how awesome he was during his elimination in the 2nd season, despite the fact that he cost the team the challenge and spent the entire episode trying to eat two of his teammates), and Character Focus (such as extended dream sequences of him naked and frolicking on mountains made of cheese). His face was even used as the icon for the first season on at least Cartoon Network. However, this decreased as the series went on, the second season giving him a lesser role and the third season giving Owen much less praise from the other contestants, and he received some decent character development with his friendship with Noah and his breakup with Izzy. The writers must have noticed the backlash, as Owen does not appear in seasons 4 and 5, save for minor cameoes.
Mike and Zoey, especially once All-Stars aired. Both characters get way more screen time than the rest of the cast on both seasons, even overshadowing the likes of Duncan, Gwen and Courtney on the latter, just to show off their relationship, without actually having much character development overall.
In-universe example from Rocko's Modern Life: while working on the "Wacky Delly" cartoon for Ralph Bighead, Filbert becomes very protective of The Cheese, the character he created/voiced.
Cheese: I am The Cheese! I am the best character on the show! I am better than both The Salami and The Bologna combined!