Someone has made a list of "Politically Correct Ways of Indicating Stupidity." Perhaps you have heard some of these. They're quite creative. Speaking of someone who has done something really dumb, we might say:
He's a few clowns short of a circus . . .
A few fries short of a Happy Meal . . .
A few peas short of a casserole . . .
He doesn't have all his corn flakes in one box . . .
The wheel's spinning, but the hamster's dead . . .
His antenna doesn't pick up all the channels . . .
His belt doesn't go through all the loops . . .
The elevator doesn't go all the way to the top floor . . .
He is several cards short of a full deck . . .
If he had another brain, it would be lonely . . .
Missing a few buttons on his remote control . . .
The lights are on, but nobody's home . . .
Some…