Here's how it plays out:
Smart People are mean and Dumb People are nice. Typically, geniuses are psychotic and high-strung, idiots are nice and sweet, because Ignorance Is Bliss. In comedy pairs, the genius will often smack around the idiot.
The smart character will be mean because he is arrogant and literally doesn't suffer fools gladly. In more sympathetic portrayals, the smart character is good underneath, but made cynical and bitter by his keen intellect, which allows him to see that the world sucks, or their lives will one day end. Inversely, the dumb character has no reason to be continuously happy, other than to provide a foil for the smart character, to show that being all smart all the time makes you unlikable, miserable or both, and why would you choose to be that way if ignorance is bliss?
This trope can also be applied to intelligence itself, with the Book Dumb (who doesn't show himself to be that dumb) hero's street smarts and practical experience being shown as superior to academics whose knowledge is dusty, dry and doesn't work in the real world, another tenet of Anti-Intellectualism. Indeed, it is always risky to assume that an uneducated character (or an uneducated person in Real Life, for that matter) will always be dumb. A character may become smart purely through private study, life experience, or just plain Awesomeness by Analysis. Simply hanging around intelligent or cultured people can help, too.
This is a pretty common trope in many films, especially Oscar Bait. Intelligent and rational people are portrayed as emotionally unfulfilled, neglectful of their family and friends, mean, or corrupt, and contrasted with a simple-minded, often mentally-delayed character with a heart of gold.
It may be caused by the This Loser Is You effect, since Viewers Are Morons; also closely related to Book Dumb. Also likely to believe that School Is for Losers. Not to be confused with Good Is Dumb. This can extend to intellectual pursuits, leading to Straw Vulcan, Science Is Bad or even Science Is Wrong. Can also lead to an Idiot Hero and/or Brains Evil, Brawn Good due to Brains Versus Brawn. Often found with a Minion with an F in Evil character.
A direct inversion of Good Is Not Dumb. Another inversion is an Insufferable Imbecile; a mentally disabled jerk may invoke the Disability as an Excuse for Jerkassery. Contrast with Gentleman and a Scholar, in which an intelligent character is one of the most pleasant, thoughtful, and well-adjusted people around. Try not to get this and Silent Protagonist mixed up, along with the Kindhearted Simpleton (whose levels of stupidity, friendliness, tolerance, and patience can relate, but they mainly differ by the Kindhearted Simpleton's qualities by their evident compassion, altruistic dialog/behavior, sympathetic communication, massive naivete, and the quality of being a genuinely simple person who has unintelligence or lack common-sense yet has an evident heart of gold and good-nature as opposed to the Dumb Is Good with just being friendly, reasonable at times, and The Ditz).
Contrast The Brute, a villain that while often lacking in intelligence can still be a threat to our heroes with their brute strength. Also contrast Stupid Crooks, which is about none-too-bright criminals, whose stupidity is meant to be mocked.
