My mom and I went round and round about me shaving my legs when I was in 8th grade. It really boiled down to literally, everyone else was doing it. Obviously that's a stupid reason to do something, but at that age, it's totally valid and makes all the sense in the world. Maybe since you don't shave (and are clearly past the teen "I care what other people think stage
Having only ever been a teen girl and not having parented one, I would try to support her. Obviously it's important to her.
Remember that you have learned from all of the things you went through with your mom. Just by being concious of the things that bothered you growing up, you will do better. I promise