I think he's a little bit of a gray area. I mean if he worked for Marvel or Dark Horse, etc, then yeah, he's a serious professional, and should be working to improve.
But he kind of "accidentally" got popular, and there's tons of people clamoring for the art he has the capability of doing now. So his target market (his book-buying fans, not his hecklers) is telling him "We like the stuff you do now", but the part of him that wants to do this as a career is telling him to get better.
I'd call him a casual professional. What that means as far as improvement requirements? I have no idea.