As far as the colors go, in your palette, the values are very close together. Aside from some pure black pixels for the eyes, you had really no darks. I put yours and mine together so you can see what values I changed and what I eliminated.
Most pixel art looks better with higher contrast. A good well rounded palette balanced with darks, midtones and lighter colors generally is the way to go. Also pixel art tends to have a smaller canvas, so there's not a lot of room for lots of different colors without looking messy.
Also darker colors make lighter colors look more vivid. That's why I used outlines. The one main yellow color I used from your palette is virtually the same. It just pops more because of the colors I used next to it.