THREE HUNDRED!!!!!!! YOU????????

or...

10 short of 300

A few screws short of a hardware store.

Not the sharpest knife in the drawer.

A few cards short of a deck.

A few fries short of a Happy Meal.

About as sharp as a marble.

Only has one oar in the water.

The elevator doesn't go all the way to the top floor.

A few peas short of a casserole.

The gates are down and the lights are flashing, but the train isn't coming.

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

An intellect rivaled only by garden tools.

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

(but we do get these)

additional such expressions welcome