You need to have more confidence in your relationship. Yes, looking like Joe Exotic will make her super attractive to everyone but she’s with you. Just because looking like Joe will make her prime fuckable hotness doesn’t mean she’ll cheat on you.
Have her get a Crystal Methyd El Debarge mullet.
There was another post where a guy grew out a mullet and his girlfriend shaved her head in protest, not thinking about the fact that the guy could just cut it shorter whenever he wanted. Make a joke about it and let her do her own thing. You may be surprised how quickly you adapt. If you still hate it after a couple of months, maybe she could consider a pixie cut. Talk it out.