They're simple, but it's okay to be simple when you're starting. To make mosr advanced sprites, reshade the added parts so they look better. A lot of the game sprites only look good as their original colors, so when you recolor them, you also need to reshade and reoutline.
Also, try and make added parts fit better with the sprite. You want to make it look like it's supposed to be there, so change the shading and outlining, move a few pixels here and there, and it'll look much more natural.