>Where do people get the idea that naruto is dumb?

From the entire original series?

Naruto is unintelligent in the first series. He's well behind the curve on his classmates, he requires basic concepts to everyone else to be explained to him repeatedly, he has no strategic mind whatsoever (when most of his peers already have some), and it's even mentioned a couple of times he sucks at pretty much everything besides 'here's a bunch of chakra'. It's mentioned several times that his testing scores are atrocious. He's also just socially clueless.

He's markedly more intelligent in Shippuden, or at the very least his ego's gone down so he listens to his intellectual betters. But even then, he's still just kind of average. It's really not until the pain fight when he starts showing real intelligence.