"Judge not, lest ye be judged"
Mark 7:21-23 "For from within, out of men's hearts, come evil thoughts, sexual immorality, theft, murder, adultery, greed, slander, arrogance, and folly. All these evils come from inside a man and make a man 'unclean'."
Slander is not nice.
edit:I didn't read anything there on someone being "stupid". Pretty sure you won't find "stupid" in the Bible anywhere as a definition of sin, either, other than a defense(and that's O.T.).