"stupid" would be rude, so say...

About as sharp as a marble.
A few clowns short of a circus.
Not the brightest bulb on the Christmas Tree.
A few fries short of a Happy Meal.
I wish I had a blueprint for his brain; I'm trying to build an idiot.
An experiment in Artificial Stupidity.
He only has one oar in the water.
A few beers short of a six-pack.
Dumber than a box of hair.
A few peas short of a casserole.
Doesn't have all her Cornflakes in one box.
One Fruit Loop shy of a full bowl.
One taco short of a combination plate.
A few feathers short of a whole duck.
All foam, no beer.
The cheese slid off her cracker.
Body by Fisher, brains by Mattel.
Couldn't pour water out of a boot with instructions on the heel.
He fell out of the Stupid tree and hit every branch on the way down.
An intellect rivaled only by garden tools.
As smart as bait.
Chimney's clogged.
Doesn't have all his dogs on one leash.
Elevator doesn't go all the way to the top floor.
Forgot to pay her brain bill.
Her sewing machine's out of thread.
His antenna doesn't pick up all the channels.
His belt doesn't go through all the loops.
If she had another brain, it would be lonely.
Missing a few buttons on his remote control.
No grain in the silo.
Proof that evolution CAN go in reverse.
Receiver is off the hook.
Several nuts short of a full pouch.
Skylight leaks a little.
Slinky's kinked.
Surfing in Nebraska.
Too much yardage between the goal posts.
Not the sharpest knife in the drawer.
The lights are on, but nobody's home.
24 cents short of a quarter.
The wheel's spinning, but the hamster's dead.

Never miss a good chance to shut up