jakamarra53 gave me this idea from BlueLady's crazy gf post here: https://kat.ph/community/show/27368/

My favorite is:
He's a few clowns short of a full circus

My two favorites are variations on a theme. Either "He's nuttier than squirrel sh*t", or "He's as nutty as a porta-potty at a peanut festival"

And then there's the one for the intelligence-challenged: "If brains were dynamite, he couldn't blow his nose."

That boy is about as bright as a burnt out light bulb.

The garden path doesn't go all the way to the shed.
Thick as two short planks.
Not enough chlorine in his gene pool.
Nutty as a fruitcake.
A couple of cans short of a six pack.
The lights are on but nobody's home.

Dumber than the box the rocks came in.
Sharp as a marble.
The porch light's on, but no one's home.
Hamburger short of a happy meal.
Not the brightest Chicken Light on the trailer. (trucker joke)

Doesn't have all his dogs on one leash.

He must have two d*cks, couldn't get that silly, playing with one

The lights are on, but nobody's home.
Thick as a brick.
Dumb as dirt.
If brain's were water, you'd die of thirst.

A few feathers short of a whole duck.

that boy jus' ain't right in the head
he's touched in tha head
his boat don't motor
he's a brick shy of a load
his bucket's got a hole in it
there's bats in that belfry for sure
dim as a busted light bulb
ain't workin' with a full deck