He's/She's not stupid, she/he's...

A couple of notes shy of an octave.

A .22 caliber intellect in a .357 Magnum world.

A day late and a dollar short.

A few bricks short of a wall.

A few clowns short of a circus.

A few clues shy of a solution.

A few feathers short of a whole duck.

A few points short of a polygon.

A few sandwiches short of a picnic.

A few too many lights out in his Christmas tree.

A few volts below threshold.

A prime candidate for natural deselection.

A room temperature IQ.

A walking argument for birth control.

All foam. no beer.

An experiment in Artificial Stupidity.

Answers the door when the phone rings.

Any slower and he'd be in reverse.

A few fries short of a Happy Meal.

Not the sharpest knife in the drawer

Not the brightest penny in the jar.

Dumber than a box of hair.

Couldn't pour water out of a boot with instructions on the heel.

Fell out of the stupid tree and hit every branch on the way down.

An intellect rivaled only by garden tools.

Chimney is clogged.

Her sewing machine is out of thread.

His antenna doesn't pick up all the channels.

If he 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.