My favorite is a quote from a child, which I read on this board:
One from my dad: “I believe he’s got one light out in the ol’ marquee.”
Ones that I tend to use: dumb as a brick and drivin’the short bus.
The only thing he / she uses their head for is a hat rack.
That boy’s got about a 15 watt bulb.
Off-key in the upper registers.
Wears a ponytail/cap to hide the valve stem.
Batteries not included.
Sharp, like stone in river. Swift, like tree through forest
The gene pool could use a little chlorine.
If you stand close enough to him, you can hear the ocean.
A favorite from my paramedic days:
Sucking head wound
You’ve got a point…but keep your hat on and most folks won’t notice.
Two tacos short of a combination plate.
Somebody please blow in her ear and give her a refill.
That guy is living proof you don’t have to be female to be a bimbo.
Lays his shingles nice and straight, but always starts at the top.
Donna: See Bob, I’m not as dumb as I look!
Bob: <mumbling> You couldn’t be.
Daft as a two-bob watch.
Smarter than a tall building
IQ matches shoe size/bust line/age
to be used while waving hand in front of face… “whew! Didja smell that brain fart?!”
Dances with stupid
Sorry, my dumbometer doesn’t go that low
Striking a burnt match
FML…
A favorite from my southern Grandma: “His momma loves him & that’s good enough for me.” (usually followed by that favorite, “bless his lil heart.”)
Love, Phil
All my favourite Foghorn Leghorn type ones have been taken, so I’ll add one I like for when a person is not dumb, but rather a little odd or eccentric:
“The dollar’s all there, but it’s all in loose change.”
Penny short of a pound.
Not the full shilling.
Numb as a pit-prop.
He’s so dumb, it takes him and hour and a half to watch 60 Minutes (or, it takes him half an hour to cook minute rice).
He’s nuttier than squirrel shit.
“His antenna doesn’t pick up too many stations!” (from an old radio broadcaster…) 
One day he’s going to make a great Darwin award.
He thinks he’s a real wit. He’s half right.
He’s a little too tall for his blood supply.
The oven’s on but nothing’s cooking.
He has an intellect rivaled only by garden tools.
He would be out of his depth in a parking lot puddle.
Not the brightest candle in the menorah.
Couldn’t get a clue if it was clue mating season, he was in a clue singles bar full of horny drunk clues, rubbed clue musk all over himself and did the clue mating dance. he still couldn’t get a clue.
Cluricaun I had always heard that as driving a piss ant’s go cart around a B-B, but I like yours better.
A couple of automotive ones:
I think he goes lean at high revs.
That boy just can’t get into turbo boost.
When they were passing out brains he thought they said trains, and asked for a slow one.
“I say, the boy’s so dumb, he thinks the Mexican border pays rent!”
Also: He couldn’t find his ass with a ten-man search party.
For all my computer geek brothers and sisters…
He’s two bits short of a three.