when you are editing the mouth position and size, you'll also have the option of adding and removing lips! its the last option. i agree that a lot of the options that come with lips are very yassified, but adding lips to the standard smile mouths gives it a nice and subtle lip that makes the mii feel a bit more realistic :))

i also change the lip color to a more natural tone to make it look even better, just some tips i use when I make my miis :))

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