Baballama is right there are many better options.
Here’s a partial list, alphabetized for your convenience.
The vulgar ones are not included.
airhead, birdbrain, blockhead, bonehead, b**b, bozo, chowderhead, chump, clod, cretin, dimwit, dingbat, dipstick, ditz, dolt, donkey, doofus, dork, dum-dum, dumbass, dumbo, dummy, dunce, dunderhead, fathead, halfwit, ignoramus, imbecile, jackass, jerk, jughead, knuckle-dragger, knucklehead, lamebrain, lummox, meatball, meathead, m***n, mouth-breather, nerd, nitwit, numbnuts, numbskull, peabrain, pinhead, sap, schmuck, simpleton, thickhead, turkey, twerp, twit, wooden-head. . . .
My favorites are the three "I's" Idiot, Ignoramus, Imbecile. They, along with m***n, actually have a formal, legal meaning.
Remember the best way to hide dumbness is to remain silent. You have the right to remain silent. Anything you say will be probably be stupid anyway.
This is not all inclusive. There are a great many words describing diminished brain/mental function. The reason for this is such defects are common, and by no means represents a minority of any standard population. Even normally functioning high-level intelligence suffers from intervals of CDD.
Hey, BabaLlama, would you like to come to dinner? You will be the guest of honor.
+1036 