Stupid:
Nob Head (but personally i prefer Nob Sock)
A few elephants short of a disc. 
i love :
thick as pig shit
and of course the tried and [still] tested putting your tongue between your teeth and your bottom lip and simply calling out someones name!
Couldn’t pour piss out of a boot with a label on the heel saying turn over.
I like “he’s/she’s touched” with the tapping of the temple for nutty.
One that was said a lot of management at a place I worked was “he couldn’t run a fevor”
fever.
NON PC
one synapse away from the special olympics.
The wheel is spinning but the hamster’s dead.
Industrial strength stupid
A few Froot Loops shy of a full bowl.
A few fries short of a Happy Meal …
A Styrofoam cup short of a landfill.
Nerfhead.
Dumb as a post.
And a joke from grammar school: “If I took his brain, and rolled it down the edge of a razor blade, it would be like rolling a BB down a four-lane highway.”
For looniness, heard on sportsradio the other day, talking (IIRC) about Tigers’ pitcher Jose Lima:
“He’s just this side of a padded room”