Another one that started out mildly idiotic and turned mildly retarded by the end of it's run was Joey on Blossom.

"The Dumbing Down of Jackie Harris and Buddy Lembeck" - news at 11.

Roseanne Connor's sister, Jackie, not only became dumber, but the actress apparently forgot how to act as well. Same can be said for Buddy, Charles' best friend in Charles In Charge.

How writers and directors think that the dumbing down of popular characters can help improve shows is beyond me.

Jon and Kate Gosselin.

I started watching Scrubs every night from Season 1--I'm on season 5 now. Elliot starts off a little off-beat, but her idiosyncrasies increase as the seasons go on for sure.

I agree about Jackie. With the exception of choosing the wrong men, she was very level headed in the first several seasons. I remember reading that the producers wanted to dumb the character down some and she used Barney Fife as a model. She also became neurotic and paranoid.

Jake from Two & A Half Men was actually a little smart the first season but is now a dumbass.
Wait maybe I am for watching this show.

Seth McFarlane

I thought of Eric right away when I saw this thread. In the early seasons of Boy Meets World he was like the wiser older brother but the in the last few seasons he was a mornon.

That's a good one. It looked like he was going to be a dorky smart kid. But now he's just a dorky dumb kid.