Hmm, getting a lot of urine-based imagery here. Oh well. Here’s another:
Couldn’t pour piss out of a boot if the instructions were on the heel.
Dumber than a ditch carp
See post #38. 
Thick as a post.
Thick as a brick.
Dumb enough to pee into the wind.
A wise man short of a nativity scene.
Isn’t that a song title by the band “Jethro Tull”? 
“When God gave out brains, he thought He said “trains”, and he’s still waiting at the station.” <------ classic insult from my childhood.
D’oh!
Thick as…well, two really thick things, I am.
If God gave him brains he’d be dangerous.
A two-dimensional mind trapped in a three-dimensional reality.
If brains was rubber, he wouldn’t have enough to make a flip-flop for a one legged budgie.
Doesn’t know his balls from Kentucky Fried Chicken.
.
I heard it as, "When God was passing out brains you thought He said trains and said, “No thanks I’ll take the next one.”
And… "When God was passing out looks you thought He said books and said, “Yeah give me a scary one!”
Too stupid to be allowed to live.
He can’t tell his ass from his elbow.
Little sign between her ears that says “space for rent”.
Bone from the eyes up.
If brains were gasoline he wouldn’t have enough to power an ants go-kart around a Cheerio.
“If ignorance was Corn Flakes, you’d be General Mills”
(What took you guys so long?)
Related political reference: Thinks Roe versus Wade is the choice you have to make when deciding how to cross a river.