The problem isn't that it's too simple. Plenty of simple pieces get submitted.

The problem is probably that the background looks jagged and unpolished, and that your piece is pre-zoomed, and unevenly at that. Submit at 1x unless you have some reason not to - the site has a zoom feature, so people can zoom in if they want.

As for the jagged lines, here's an edit that uses regular lines to show you what I mean:

If I kept all the line segments the same width in each line, this gives them a cleaner look. I also tried to make them look like they're radiating from the character.
I also made the character a little rounder because their circle shaped looked jagged/uneven to me, but I might've misinterpreted your intended design. Feel free to ignore that.