"Is Ash Smart?"
The answer to that depends on who you ask. Some people think he's a clueless nut who wins battles on luck and hAx all the time, others think he's become a reasonably competent trainer prone to making rookie mistakes, still others think his intelligence level depends on how the writers want him to be portrayed in any given episode.
As for the whole legendaries thing, you realize they're not SUPPOSED to be captured in the anime, right? Did you watch any of the movies? Most, if not all, of the legendaries have some sort of importance in maintaining balance in the world, whether it's in nature or the fabric of time and space. There would be no planet left if you went around capturing all of them, and that's assuming you even can in an actual Pokemon battle.