Like ur other icons of the same kind, this one is pixelated too.
I also use Gimp, and my drawings don't come out pixelated, unless i u se the sharpen tool in an exaggerated mode.
So, assuming that's work from scratch, it shouldn't be pixelated(imo), unless u over-sharpen it.
The icon is kinda dark, there's too much black.
Coz the ball is black, u can change the bg, to improve the atmosphere.
Shading/highlights also need work. Regarding this, u can see some sphere drawing tutorials.
The horns color could be another shade of yellow, a bit more vivid yellow, to break the monotony that dominates the icon. Those horns also need proper shading.
U can even do a bit zoom out.
Its feet and that glow are both reddish, it's confusing.
Some pics that might inspire u:
http://www.mariowiki.com/images/e/e9/MarioBully_SM3DW.png,
http://www.galeantokal.com/work/1997/