Hi and welcome to the PixelJoint forums!

Please do not post upscaled art on the forums, a pixel should measure a pixel, not more. There's a functionality that allows to zoom in on any image by clicking it and zoom out by using CTRL+click.

A great way to find mistakes is to flip your piece horizontally (Or vertically for really challenging fixes), here's the flipped version of your piece.



Here are my thoughts on what could make the piece look better:

      - Get rid of the dithering, it seems like it's doing more harm than good to the piece

      - Pick up a position for a light source and render your piece accordingly, as it is shadows and highlights seem to be putted at random. Every plane that faces towards the light should have the lightest values, every plane that faces away form the light should have the darkest values. Specular only happens on planes that reflect light from the lightsource directly to the viewer.

      - The rainbow effect have colors swapped out of place, in the light spectrum the color next to the yellow is green, not blue. So the correct order for your piece should be red, orange, yellow, green, blue.

      - The character seems to have structural issues, specially on her limbs, look at reference when drawing, improvement is almost immediate.

      - There are 36 almost identical colors with alpha on your piece, get rid of them, this is seen as a mistake on pixel art.

Hope this helps!