He has a room temperature I.Q.
A traditional Spanish one I found myself thinking at someone today:
Aaaay, ¡alma (de) cántaro!
Conceptual translation: Brains like a mason jar!
And no, cántaro doesn’t mean mason jar 
“Alma” means “soul”, “that which animates, i.e., which gives movement and thought,” but also the center of something that’s got any kind of tubular structure: the center of a tree’s trunk, the sword inside a swordcane… or the empty space in the center of any kind of storage container (in this case, a chubby, earthenware pot).
In Thai we compare someone with /sǎam sà-lʉ̌ng/ : three ¼-bahts, not a full baht.
“A couple of French Fries short of a plate of poutine!” (Roger Abbot, Royal Canadian Air Farce)
“If you packed all your brains in a pollywog’s head, he’d never ever grow to be a frog!” (Shirley Jones, Carousel)
::spoken in an Og-like voice:: Strong like bull, smart like dumptruck.
When they were handing out brains, s/he thought they said trains and asked for a slow one.
I can look at that list, it breaks my browser.
Is the phrase “motor is running, nobody at the wheel” or similar on there?
I’ve always liked “not the sharpest taco on the combination plate”.
And from Blackadder the Third, “thick as a whale omelet” is pretty good too.
Dumber than a ditch carp.
I will begin by admitting that I did not read each and every post.
Bright as a small appliance bulb.
Our version was “Strong like bull, dumb like steer”.
Have we yet had “The lights are on but nobody"s home”?
I think posting a list like that defeats the purpose of the OP, and takes all the fun out of it. I for one am ignoring it.
“He’s three lines shy of a haiku and they don’t even have to rhyme.”
He’s psychologically inert.
Can’t think and breathe at the same time, you can always tell when he’s thinking, he’s lying flat on his back and blue in the face.
Dumb enough to drink their own bathwater.
Heh! I use that one occasionally. My memory is pretty shitty, but if pressed, I’d say I probably picked it up here. Y’all are (as a whole) much wittier than most of my meatspace interactions.
One I use, although it doesn’t follow the standard format of most prior posts, is to say very softly, and with much sadness in my voice, “That poor lil ol’ brain cell. Rattlin’ around, all by it’s lonesome in there.” Helps to kick up the Southern drawl. Usually gets a laugh.
Ain’t got the sense God gave a sweet potato
Well bless your heart. 
Third from the left on the Darwin Chart
I’ve heard
“doesn’t have the sense God gave sheep”
and my Dad used to say “half a bubble off center” but I think that means odd, not necessarily dumb.