This really isn't supposed to insult a character. Who would do the worst on an IQ test? The rain man is a genius, but would probably suck at an actual IQ test. That is my reasoning for some people on the list.
http://www.ranker.com/list/movie-characters-with-the-lowest-iq/stokified,
Frankenstein's monster
Bob Wiley
What About Bob?
Landfill
Beerfest
Joe Dirt
Joe Dirt
Forest Gump
Forest Gump
Carl Spackler
Caddyshack
Navin R Johnson
The Jerk
Bubba
Forrest Gump
Bobby Boucher
The Waterboy
Ishmael
Kingpin