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?
Brian
Half Baked
Scarface
Half Baked
Kenny Davis
Half Baked
Landfill
Beerfest
Forest Gump
Forest Gump
Frank Drebin
The Naked Gun
Bobby Boucher
The Waterboy
Ishmael
Kingpin