I thought the accident he met with when he was a kid is what made him dumb well ignorant. where he falls from a mountain cliff otherwise he would be ruthless like all saiyans. For the record i am a fan of goku.
Thing is, how many Saiyans (pure bloods) are actually smart though?
Nappa was stupid and had to have Vegeta coach and command him during the fight on Earth.
Radditz was a beast who killed and ate meat raw and was snared and killed by the same two people twice, with the same move.
Goku was a wild child, but he's never been much on brains.
Bardock was warrior smart but not much else.
Frieza, Ginyu and Zarbon all disparaged the Saiyans as being a bunch of dumb monkeys, which isn't entirely inaccurate. The Saiyans didn't build or create anything. They were space pirates, who traded their fighting skills for money, land and technology.