For me, it looks as if you used a bucket tool, and drew everything in a single layer. The base colors are messy, and the base colors as well as outline is jagged, thick and weird. Sorry if this comes off as rude, but it’s an honest opinion.
As for advice, I suggest avoiding the bucket tool and using many different layers for each base color. A new base color layer for the skin, mouth, hair, eyes and clothes. Use a 3.5 sized brush for the outline with a canvas of 2000x2000, I use 4.0 for the hair.
Try to avoid drawing lines slowly, this makes it jagged and curvy.
Example:
The top line is drawing in a slow manner and the bottom a swift line, notice the difference.
For shading, make sure to use the clipping tool
Example:
(Sorry for the terrible outlining and shading lmao.)
Notice how I separate the outline for hair and face, as well as the base color and shading.
I think this is what I have to advice for now, hope it helps. 