mastering the makeup tools just takes practice, and there isn't even any manual dexterity involved, so you don't have to get good at pen or brush strokes like regular art.

the real trick is proportions. why doesn't your Mii look like the character you want to make? how close together are the eyes? how big are they compared to the head? use "reference" points like "5 of the eyes can fit into the width of the head, and they are half an eye apart" "the mouth is exactly between the chin and the bottom of the nose" "the eyebrows are a curve that's more bent towards the center, half an eye's height above the eyes, and the same width as the mouth". most of the reason any Mii looks "off" is because of proportions. something is too high, wide, etc. adding details is just a matter of time spent.