Naruto does lots of stupid things but he's only been outright defeated in a life or death fight twice (Kabuto and Sasuke). Every other time he either wins his fight or something interferes and the fight is not really settled but he ends up no longer fighting that opponent.

Naruto has demonstrated that he's a very poor learner unless it's through experiences.Because he's experienced losing only once (I believe he passed out thinking he'd beaten Kabuto so he probably didn't learn anything from that) he hasn't really been forced to reflect on his performance after a defeat and think of ways to improve his skills.

Sure it's stupid for him to do the first thing that pops into his head? But it generally works out fine for him in the end and that's all he really cares about. The downside to having infinite confidence is that you never think you need to improve.