Mr VOW and I were bantering about a certain famous jerk who has exhibited for the hundred-thousandth time he is lacking in his mental operations.
We came up with a few standards:
He doesn’t have all the raisins in his toast.
His porch light is flickering. (Mr VOW corrected this to “His porch light is burned out!”
He’s a few tacos short of a combo plate.
C’mon, let’s poke some fun. If I have to see the Capitol invasion tape again, I might throw up.
~VOW
The lights are on, but nobody’s home.
Some standards:
A few inches short of a foot
Not firing on all cylinders
His/her elevator doesn’t go all the way to the top
A few knives short of a drawer [or something like that]
I always want to make up good variations, but can’t think of any good ones I’ve personally invented. If I can think of any, you know I’ll share.
Not the sharpest knife in the drawer.
A toy short of a Happy Meal.
His computer is lacking a CPU.
gone off-trail again
dumber than a bag of hammers
Not the brightest bulb on the marquee.
A few cards short of a deck.
They’re the kind of person who, if you blew in their ear, would say, “Thanks for the fill-up.”
Cobwebs on the windmill of his mind.
He’s got a few loose screws, or maybe a lot and some are even missing.
He went out to lunch and stayed for happy hour.
He’s about as sharp as a bowling ball.
He has a room temperature IQ.
His bus has jumped the track.
He marches to a different kazoo.
She keeps losing her left socks.
She can count her brains cells on the fingers of her left foot.
A few fries short of a Happy Meal (the variant I heard)
Couldn’t find his way out of a paper bag with a map and a flashlight.
This is a lot more insulting in places that use the Celsius scale. 
Couldn’t pour the water out of his boots if the instructions were printed on the heel – from the military?
Here’s a whole website full of these: Cheap Thoughts
As sharp as a bag of wet mice.
Related:
Dang, Sunspace! You beat me to it.
Low as dirt and half as smart.
Numb as a pounded thumb
and, my favorite from a former boss who was famous for screwing up sayings and misusing words in general:
A few bricks short of a picnic
Another child left behind.
(Too many people don’t get this one, but I think it’s hilarious.)
A few bricks shy of a full hod.
… bag full of doorknobs.
Couldn’t find his rear end with a six-man search team.
Dumber than a ditch carp.
Suffering from microdeckia