So… do you think Luffy is smart or dumb? He's shown a little bit of both to me.
Normally, Luffy acts simple-minded and doesn't understand the situation at all, while even the idiots of idiots could wrap their head around it.
However, he's also able to detect subtle weaknesses in his opponent's. He found out Crocodile's weakness, while even the smartest and strongest warriors were like ants before him. That's pretty amazing.
Aside from this, he learned Soru from Bleuno. Not only that, he was almost a master at it from the start. He overpowered a Rokushiki user, using a technique he took 5 minutes to learn, while the CP9 had practiced and mastered their entire lives.
He can be a genius to geniuses, and a complete retard at the same time.