The one thing you should try out is shadows (and light).
Look at something like the Sprite Sheets for Samus from older Metroid games, shadows are essential to make your art look three dimensional... simply put, it gives that "professional" touch.
(High)Lights do the same, but it might be easier to think about shadows on the body first.
If you want, take a look at this animation to compare your work to: https://twitter.com/i/web/status/1403297496927858695
You might want to look for an Pixel Artist to study their usage of shade to apply to your art, until you develop an understanding of your own.
Also, if you think something is not right with your art, give it another try. You will usually do better on the second try, especially if you are doing the exact same motif.

It's a cute design, by the way.

Oh, you could also try adding contour lines! That way it is a bit easier to distinguish the forms without adding any shadows.