Leveling outfits seems to be simple. Using the outfit and unlocking more of them by collecting XP for the others.
When I unlock the 2nd outfit in the row it works fine. But leveling the 3rd Outfit isn’t possible when the 2nd outfit is equipped. I have to wear the first.
Russ told me on Discord that that isn’t an expected behavior and I should open a bug report here… So here you are 