Worst part is, that wasn't a studio mandate... that's what the actual creators wanted. They saw a character they created... nay... created and one of them even does the voice for, and they picked an actor that really couldn't pull it off.
Love or hate or inbetween Jim Carrey, he just doesn't have the comedic style to pull Doof off. He's far too hyper, and incapable of going on long weird rambles and large hammy bouts of anger. I'd say Steve Carrell would be an excellent Doof, but he already has a sympathetic villain with a pointy nose and mother issues.