Spark's Sprites

[Gors]

before going any further, a lot of characters there could use a better balance. To test the character's balance, flip it horizontally: it'll reveal balance errors. Calcifer's portrait is obviously blurred, there's too many shades going on. Do it with a limited palette, or else he'll clash with the others' levels of detail. Princess Mononoke is actually a lot taller than Porco Rosso, if you aren't aware. She is in a crouched position, and if she was standing normally, she'd be taller or same height than Baron.
Kiki's portrait looks weird. Her facial features aren't evenly placed.

Maybe your problem here is not imitating Miyazaki's style. It looks like his drawings, honestly. Your problem, though, is trying to port it in a different media with the same techniques. Pixelart is a low-resolution art media, so along with colored outlines, you should use anti-alias to smooth out the lines to make it look like a real drawing. Anti-alias doesn't occur in drawings because it's a traditional, high-definition art. But when working with small pixels, you need to use additional techniques to ensure the smoothness. I'll make a sprite of Kiki to exemplify what I'm trying to say.

I know I said this kinda late but don't worry, as this can be fixed with some minor edits here and there. My sprite might not be ideal either; it's just showing how I'd make her. You can ignore it, but it'd be cool if you studied it for techniques.