"He's dumber than a bag of hammers."
A hammer is an object with no intelligence. If a hammer was a person, you might see it as a stupid person, because it does a very simple job.
If you put a lot of hammers in a bag, you could say that the bag was full of stupid things. Many hammers together are stupider than one hammer.
"He is more stupid than something that is very stupid."
"Dumber than a box of rocks" is a variation of this phrase.
Here are some more.
Student or Learner