Good point about art that complements the humor - I'd also include xkcd and Dinosaur Comics in that category. I think another part of it is that those comics' authors have great confidence in their artistic choice - they've chosen that art style because it works for them, not because they're trying to imitate other, technically "better" art styles.
When I think of low-quality art that prevents me from enjoying a comic, what usually comes to mind for me is someone who knows and clearly wants to draw in a specific high-quality style, but doesn't have enough skill to pull it off, causing their art to feel like an awkward imitation. For example, a comic whose style tries to imitate Western superhero comics, but none of the action poses are anatomically correct, and muscle definition feels more like random lines. Or a Japanese-influenced comic whose characters' expressions are so stereotypically "manga" that it takes me out of the suspension of disbelief that the characters would actually feel that way. I guess I'm much more willing to accept a comic with an unorthodox style, even if not technically skilled, than a comic that imitates technical skill poorly.
(None of this is to say people who want to draw well in a specific style, and don't currently, shouldn't keep using that style in a comic - how would you improve if you don't draw? My own comic is in that category. But it's something I'm really attentive to and can't just ignore myself.)__
