Naruto is not stupid.

proof :

stupidity multiplied thousands of times is still stupidity.

Naruto multiplied thousands of times is rasen-shuriken, which is a move that could not be mastered (created?) even by 4th.

Naruto just pretends to be goofy cause it's easier for him, he likes to be underestimated and surprise his enemies, who would expect him to be dangerous at first sight? pretty much nobody