In the intro to his script of Chasing Amy, he described Clerks as over-praised, and Mallrats as over-criticized. I concur with both of those things: Mallrats isn’t great, by any means, but the barrage of criticism had as much to do with failing to fulfill the promise of Clerks as anything else.
But while Clerks was a fantastic movie, given its budget, it also had a lot of flaws. Smith is at his best as a screenwriter; he’s sharp and insightful. At his best, he’s outrageous in a novel way, talking about uncomfortable subjects with a deadpan curiosity, or interlacing profanity with deep insight. At his worst, he’ll just be gross.
He’s a lousy director. He can’t handle a camera very well (he seems to rely on his cinematographers to compose the shots) and he doesn’t seem to know how to tell an actor what to do. When he’s lucky he’ll get great actors like Matt Damon, Ben Affleck, or Jason Lee. Or he’ll fine a gem like Jason Mewes or Jeff Anderson, mediocre talents who have great screen presence. In either of those hands, his prose really pops. If he has to explain it to the actor, it’ll just lie there.
Small, personal scripts like Clerks or Chasing Amy show the edge of brilliance. Mallrats gave him too much budget and not enough self-control; as his own editor he clearly couldn’t bear to part with things that didn’t work and didn’t know how to tighten the pace. Dogma should have been better; it was a great script that couldn’t ever get its feet under it. Jay&Silent Bob Strike Back was too self-indulgent, though funny enough if you were in its target market.
He finally, finally began to actually capitalize on his promise in Clerks II. He revisited characters he really understood and knew how to find the funny. He filled it with seasoned actors and the gems he’d found earlier. And I think he finally found a cinematographer who worked for his style, and he finally learned how to edit himself properly.
I don't read comic books, but I'm told that his work on X-Men is fantastic. And I believe it: he's a great writer, as long as there's somebody else in the loop on the storytelling.