Compared to almost any artist, ZUN's art is terrible. Compared to the average person who doesn't draw much (probably the remaining 70% of the population), he draws better than they would. Which isn't really bad for a guy who (maybe) said he doesn't care for drawing much, and would almost positively rather be spending that time programming, writing, character-designing (not to be confused with actually drawing the character), danmaku-designing, and composing.
ZUN's already a great programmer, composer, designer, and writer. No one can be perfect in EVERYTIHING, I suppose.
At any rate, to elaborate on one of the above, his character designs really are good. Even with his bad art, there are tons of touches in them that shows just how much thought he put into the character designs, themselves, all over the place. That's probably one of the major reasons fan artists love to draw his characters. Sure, his art of them is not so hot, but the base design tends to be fantastic, filled with all sorts of themes, shout-outs, mythology allusions, and style (and that's not even including all the thought he tends to pour into danmaku designs for those characters). While he might not draw as good as any actual artist, he certainly designs great original characters better than even most artists would..
