To separate characters into categories that have seemingly come up here, I will utilize the following 4 crappy terms:
Intelligent Intellectuals:
Lisa seems to be considered an intellectual on the show, and I would believe somewhat intellectual in that her thirst for knowledge is more cerebral, whereas Martin Prince seems more driven to succeed for the sake of success. Lisa is obviously book smart, and 'intelligently intellectual'. However, Lisa is rather lacking in street smarts, so she seems less well rounded than she could be. Mr. Smithers and Principal Skinner would fall in this category as well, I believe.
Intelligent Non-Intellectuals:
Bart is considered a non-intellectual in that he does not apply himself in school, but I do not believe I would consider him unintelligent. His capers tend to lean on the more cerebral, albeit devious, side of things. Bart may be less book smart than Lisa, but not completely unintelligent as he possesses a certain street level of intelligence that is evidenced by his chicanery. Ned Flanders might fall in this category as well, since he is not necessarily seen as a great intellectual force, yet seems rather intelligent- although naive.
Unintelligent Intellectuals:
These I would consider to be the people who are seemingly intellectuals, but are actually morons in intelligent peoples' clothing. Mayor Quimby would to me represent the seemingly intelligent buzz-word-using guy who really does not know which way is up. Quite appropriately, he is in public office and considered a leader of the community. Kent Brockman would fall in this category for me as well.
Unintelligent Non-Intellectuals:
AKA Buffoons. These are the comedic relief people we all know and love. Obviously, this would include Homer. Ralph Wiggum puts the idiot in idiot-savant. His simplistic view of the world is refreshing, but an intellectual he is not. Face value is almost too much for him. His father is shortly behind him in buffoonery.
Ah, poop. I'm bored at work. This seemed like a good idea when I thought of it... but now it just looks unintelligent & non-intellectual.