Dumber than rubber doughnuts. (This was used, I think, in Rolling Stone to describe the music of Kiss.)
The porch light’s on but nobody’s home.
The hamster died, but the wheel’s still spinnin’.
As numb as a pit prop.
As thick as pigshit.
I usually say “dumber than a sack of hair” or “dumber than a box of rocks”
You must be as smart as this fern to read this post.
Dumber than a bag of spatulas.
Dumber than a football bat
Dumber than a box of frogs.
Not the sharpest tool in the shed.
Mostly - Dumber than a door knob.
Although I’ve heard it for just about any group of people you can think of.
Dumber than a (group here).
I’ve heard that as “Queer as a football bat.”
I use “Dumber than a box of rocks on a sunny day.”
For some reason, that “sunny day” always gets me.
Dumb as a post. A simple, straightforward classic. 
“Dumb as a mailbox”
but I’ve always really like the visual of “dumb as a box of hair”
There really are never enough opportunities to quote Foghorn Leghorn:
“I say, I say the boys about as sharp as a bowling ball”
I also go for “dumber than a box of rocks” but I then add “sedimentary rocks, because igneous rocks are too sharp.”
Courtesy of the Animaniacs, I prefer
“Dumber than advertised.”
Dumber than a box of dead clams.
dumber than…
… a land war in Asia?
“Stupid as a fried popsicle” always makes me laugh…
As dumb as a chocolate fire-guard.
Depends on the audience.
Dumber than…
a bag of hammers.
a freeper poster.