I would agree with you but then we would both be wrong.

Sharp as a marble, that one

One of my personal favorites:

"They only got two brain cells and both of them are fighting for third place."

Couldn't pour water out of a boot with instructions on the heel.

Ahh yes, “it’s better to be silent and thought a fool than to speak and remove all doubt”. A classic.

In Greece we say "when it was raining brains, you had an umbrella."

Now I know what Douglas Adams was talking about.

“A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.”

You could hide your own Easter eggs.

I think I saw this one here previously “You aren’t the biggest idiot in thel world but you better hope they don’t die”.

My teacher said I was as "quick as a glacier"...

It's impossible to underestimate you.

You're the reason we have warning labels.

"If you ever had a clever thought, it died alone and afraid."

He's so far behind he thinks he's first.

If you were half as smart as you think you are, you’d be twice as smart as you really are.

A village somewhere is missing their idiot.

His train of thought is still boarding

The wheels spinning, but the hamsters dead.

He needs to carry a plant to make up for the oxygen he's wasting.

I can explain it again, if you'd like, but I can't understand for you.

"I'm guessing you weren't burdened with an overabundance of schooling."

Unencumbered by the thought process.

Saw two coworkers going about a task in the worst way I could imagine. I said "I believe if there was a harder way to do that you two would find it."

They didn't get it.

Once heard someone say "Well he's about as sharp as a sock full of soup".

You could blow in their ear and they would say thanks for the refill.

One of my all time favorites is "More foam than beer."

"At this point, you can only impress me."

"Well pardon me, ma'am but what you don't know could fill a warehouse."

If your brains were dynamite there wouldn't be enough to blow your hat off.