Why? It's true. Where is Joss Whedon's "Jersey Girl" or "Chasing Amy" or "Mallrats"? (By the way, I like Mallrats, but it's not a good movie.)
Whedon's stuff is fairly unique, always well done, and tells an awesome story. He blows Kevin Smith out of the water. I'd say that Kevin Smith's only real success as a director was Dogma. Sure he's successful, but his movies actually suck.
Dogma is actually a perfect example of what I'm talking about. Look at any scene where something physical is going on. The angles are flat-on like you'd do for a conversation, the movement is awkward. He's a fine writer, but he doesn't know how to tell people how to move or how to frame a shot for much besides people talking. And he does that well, but if you're going to film stuff beyond just talking, you need to do better. Quentin Tarantino knew this and when he was working on Kill Bill, he brought on someone to supervise him. Only then did the supervising guy (I cannot remember who he was, might have been another director) told him "Why am I here? You already know what you're doing." He would not have said this to Kevin Smith.
(Note: I'm not putting Joss Whedon over Tarantino. But ...)
Look at Whedon's stuff. He knows how to frame a conversation. He knows how to frame an action shot. He transitions well between these. When people need to move, it's clear they've been given prompts and ideas about how to do this. Whedon's also had to work with people who aren't his friends and can't just wing it that way. Direction is about bringing the script to life, cinematography, organizing the production, working with the actors, capturing the atmosphere you want. I would never say Joss Whedon is a perfect director, or a perfect writer, but he's a better director than Kevin Smith. Hell, Kevin Smith admits he's not that great a director. He knows it, which is why for a while he worked primarily in his wheelhouse, only stepping out of it for stuff like Dogma, and even then only slightly.
Side-note, I thought Chasing Amy was supposed to be Kevin Smith's best movie? I haven't seen it, but that's what I've been told by many people.