Jar Jar is obviously one.
Topic on Category talk:Characters with bad voice acting
Which characters had bad voice acting?
Diesel 10 (CGI) and many other characters from CGI Thomas and Friends.
That's totally a character with bad acting.
Jason Dohring as Terra in Kingdom Hearts Birth by Sleep. Except Terra's not a bad character.
Oh, and Gord Brody's is just awful. We need to create a page for him, by the way.
The way Tommy Wiseau portrayed Johnny resulted him in an odd voice.