Not that bad. You should try and add a person in there and make a pose from blender if your comfortable with that. But, that is great for a beginner icon designer.
Great job on your first GFX, it looks better than my first one. I recommend you try to practice using blender instead of Paint.net, though it may seem harder at first there is just so much for functionality.
Nice job on your GFX! I like it too and I do agree with everybody in the comments. I may not do GFX but I’ve had people in the past come in and out to create some for games I work on. People mostly use Blender to make there GFX, or import things from Roblox like characters, or a building if you wanted to make a cafe style GFX, Banner, or Icon. But good luck!