Yeah, I would assume that if it's something that is arranged, you wouldn't be "dressed to the nines" for the fight. However, if it's something that is spontaneous, I would think what they're wearing isn't always on the mind at that moment and whatever happens, happens.
I could also see though, not sure how likely this is, but if the 2 ladies start looking each other over and sizing each other up, noticing what the other is wearing. If one sees that what she is wearing is better or more expensive than what the other lady is wearing, maybe taking that off so the other woman doesn't get the pleasure of possibly ruining her nicer outfit and vice versa.
Like I said, not sure how likely that idea is, but I wouldn't doubt it has happened or gone through the mind before.