Downplayed: Bob's skills improve... but not by much.
Justified: The webcomic is about a universe still being under construction by a god; with time, the universe looks better and better.
Inverted: The art quality actually declines over time.
Subverted: The art quality gets better... but it's just because Bob took a holiday break and the webcomic is currently being drawn by his talented friend.
Double Subverted: But when Bob returns to his webcomic, it turns out he learned to draw better during the holiday.
The characters travel back in time to the beginning of the webcomic and end up in a crudely drawn world. They stand out so much that the locals revere them as gods.
The characters go back in time hundreds of years before the beginning of the webcomic, and end up in a world of primitive stick figures drawn in MS Paint.
Late in the comic, the characters notice something that was supposedly in sight all the time, but the crude art made it hard to make out before.
Zig Zagged: The art style shifts back and forth between the old style and the new style.
Averted: The art in Bob's webcomic doesn't change much.
As Bob draws several pages each week, he naturally becomes better at drawing from all the training.
Bob's been criticised for poor artwork, so he set out to improve his drawing skills.
Lampshaded: "Remember when we used to look completely different and disproportional?"
Bob treats his webcomic mainly as a means to practice drawing.
In-universe: the characters cannot read a blurry clue, so they wait until the art gets more detailed so that they can make out what it says.
Exploited: A fan of Bob mocks his art style for being terrible,Bob is then motivated just to spite the fan.
Defied: Bob is aiming for Stylistic Suck and tries to make sure that the art doesn't get too good.
Discussed: "I just wonder how are we all going to look in a few months, after the next mysterious Visual Upgrade event hits."
Conversed: "This webcomic may look bad, but the art is gonna get better if it is updated so frequently."
Bob invests so much effort into his art that the storytelling suffers, and eventually Bob has to quit the comic altogether because he is such a perfectionist that drawing each episode takes too much time.
Reconstructed: Bob finds an art style for himself that looks nice, yet is also not too hard to draw.
Played For Laughs: "I've been in this comic for ten years, and I still look like a stick figure drawn by a four-year-old while Bob and Alice have turned into the most realistic people in the comic! GET ME SOME ART EVOLUTION, YOU IDIOT ARTIST!"
Played For Drama: A character's wife ceases to love him after Art Evolution makes it clear that he's ugly as sin.