This is probably a stupid question, but does anyone know how to make a breton not look like he has that weird mouth...?

Use the face editing option available to change the face, and change what mouth the breton has. Not hard.

Bretons are all short and pudgy little goths.

Their mouths are generally puckered in that "I care about nothing" goth pout.

The worst is that they all seem to have the racial ability "Furrowed Brow", which cannot be removed no matter what you do.