For me it was Lisa Goes Gaga. It was focused entirely on Lady Gaga who played herself and was awkwardly inserted, I don't mind the occasional brief celebrity appearance or when they lend their voice to other characters (ex. Meryl Streep playing Jessica Lovejoy in Bart's Girlfriend) but they started to feel so forced. I tried watching the occasional episode afterwards but people like JJ Abrams would show up in Do PizzaBots Dream of Electric Guitars? and I'd lose interest.
I have started watching the show again but I am curious who else has lost interest due to an episode or trend especially those who have been watching the show since its beginning.
The term Flanderization exists because of Ned Flanders. The Simpsons literally ran so long it's characters became caricatures of themselves.
That's a really cromulent point!
I think this doesn't need to be inevitable, though. I think it's again a result of writer turnover. There were original people with a relatively cohesive vision they could write towards, maintaining a consistency against an imagined character with traits that maybe were never explicitly described (yet).
As the torch is passed from writer to writer, team to team, some of the "hidden values" that the characters behaviours orbiteded around get lost. New teams need to invent their own based on an imperfect view. It becomes a fax or a fax of a fax.
So... Maybe I'm oversimplifying it, but I suspect that writer consistency can stave off Flanderization.