Also: Just because something sells well doesnt mean the art is suddenly better. People can buy it for the story and still find the art to be horrible.
Exactly, it is more about whether it is interesting or not.
Gintama and Beelzebub don't have good artwork, but they have good sale. Furthermore, the original One Punch-man has really bad art, but it was popular in the internet world. So, the artwork doesn't really determine the series' popularity.