Has his solar panel on the south (side of the house)
Of course northern hemisphere dopers should reverse the orientation 
A 10-watt bulb trying to compete with Las Vegas.
Check out the ‘Subtle Stupidity’ links on this page
Too much sheetrock, not enough studs…
“A few too many birds sitting on his antenna”
"When they were handing out brains, he thought they said ‘trains’ and he said ‘I’ll wait for the next one.’ "
“His elevator doesn’t go to the top floor.”
And the first one I ever read, from Robert L. Fish in the sixties:
“You’re playing bridge with a pinochle deck!”
Couldn’t pour water out of a boot with instructions on the heel.
Not the brightest bulb in the string.
Not the sharpest tool in the shed.
3 PB&J’'s short of a full picnic basket
I’m partial to the Bloodhound Gang’s “not the sharpest peanut in the turd”, but I more often use my own “not the sharpest tine on the spork”.
Couldn’t pee a hole in a snow-drift.
A few kittens short of a full sack…
Not the brightest porch light on the block.
Sharp as a bowling ball.
Strong as an ox, and just about as smart.
Built too low to the ground; all the fast ones go over his head.
Most of these are Foghorn Leghorn-isms, but due to Molly Ivins, we have “Dumb as a Dallas banker”.
Apologies to any actual Dallas bankers out there. 
Not the brightest bulb on the christmas tree
Dumb as paint.
(Thanks M. Doughty)
how about
“s/he has a few kangaroos loose in the top paddock”
“dumb as a box of hammers”-thanks Jill Sobule
“when they were giving out brains, s/he thought they were giving out milkshakes and asked for an extra-thick one”
Heh, and in homage to the moron I met at a pizza joint the other night…
“She’s as thick as two deep pan pizzas with cheesy crusts”
Max
The lights are on, but no one’s home. And there’s a walrus in the garage.
And of course, we’ve all heard of “Born with a silver spoon in his mouth.” So how about, “Born with a salad fork in his ear.”
She’s a billion dollar airport with a 10 cent control tower.
A bottle short of a six pack