First on this list is the lovely Hilary Banks: from Fresh Prince of Bel-Air
She used to be a snobbish rich girl who spent time fraternizing with celebrities in trendy and shallow environmental charities. She was a bit of an airhead, but not to the point she was dumbed down in later seasons. (As you can see on the link on Hil's name.)
Our next victim is: Eric Matthews from Boy Meets World
Ah! Poor Eric! From Cool Older Brother to... a buffoon with a weird Feeny Call!
Here's the guy who gave Topanga some sound advice on relationships when she had a crush on him turned into an idiot who can't even play with sparklers. He was the first character I thought of when I asked myself: "Are there any other characters dumbed down worse than Hilary?"
Next on our list is what started out as a Paris Hilton parody... give it up for London Tipton! from The Suite Life of Zack and Cody and its spin-off Suite Life on Deck.
As I said she started as a Paris Hilton parody, but progressively became dumber and dumber... (another victim of bad writing syndrome) While the character has layers (The rest of the characters would like to add of Dumb), she is mostly played for laughs on how dumb she is.
Next on our Mostly Disney list... Starting to notice a pattern here... Is a youngest sibling. Give it up for Max Russo from Wizards of Waverly Place.
He is the youngest of the Russos but as he grew up he's become dumber than he was on season 1... or is it that he hasn't become any smarter since then? Whatever the case, my point still stands. Those antics are cute when you're 10 but not so cute when you're 13... Keeping him down is not a sign of great writing.
Next on my list is one famous cartoon character whose catchphrase is well known... D'oh!
I mean come on! They HAVE dumbed him down over his 20+ years on TV... I rest my case.
I know that most of these are kids shows, but that does not excuse them from bad writing. taking a 3d character and exploiting one quality turns them into a 2d caricature. (Problem is that this tends to happen to secondary characters. Homer's case is strange because he was dumbed down around the time HE became the star of the show... and he's a 2d caricature... D'oh!)
Peter Griffin did not make this list because his stupidity was there since the beginning...