Visually clear + fits the tone of the story = generally good enough for me!
However, if I'm not super into the story, the bar for the art may rise, like what @Rhonder said.
There was one interesting exception-ish case, though. There is a Korean comic on Naver that I started following. Interesting premise, believable characters, simplistic but still fitting art. But even though the story didn't drag, I kept losing interest. Why? Because the pacing was very slow. Intentionally and artfully slow, but for such slow pacing to work, I personally needed more complex art style.
If this comic had a more complex art style, I could've savored the art, and the slow pacing would've really worked. But as it stands, the visuals are too quickly digested (pretty much instantly) while the writing takes its time. Within each panel, the art style fits the tone. But when moving from panel to panel, the different digestion time between art and writing just didn't work for me. I've dropped the series.