I'm not sure this issue even gets to the heart of the show. Time and again we see Steven inadvertently causing problems through his inexperience and childishness, but it is also that nature and his ability to step up big time when it matters the most that ends up saving the day in the end. Often Steven needs to be carried, but he also gets ideas the Crystal Gems wouldn't think of and is making himself important to the harmony of the group. I think he does as well as any goofy kid his age could be expected to do.
But yeah, I imagine that in time he'll learn to do some stuff with his powers.
I would suggest that it's not the medium, but the quality of perception and expression, that determines the significance of art. But what would a cartoonist know? -Bill Watterson