I too was a Mom with a teenage daughter, and whilst I liked her clothes etc, I never wore them, might buy myself something similar, even the same so long as it was not the same colour as my daughters top. Why? because I would not even think about robbing my daughters own identity. So I fully understand how you feel and your Mom is wrong.
It's a wonder she has not asked to go out with you and your friends - in my mind your Mom is totally wrong and doing you a big injustice, I can well understand where your coming from. Here's a funny story about daughters and Moms (my own). I am now 60 and a grandmother too. And a few years ago my daughter and I were just talking one day. Talking girlie or womanly things etc and the subject lead back to when she was a teenager and I the Mom. We're from the UK. But in 1982 to went to America and of course went to Disney Land. Jacqui was just 16, her brother Brian 14, and I a ripe old age of 37. We had one of these 3 day passes into Disney Land, and on one of the days as we showed our tickets and went to walk through, my daughter had been behind me and had been making eyes at the Entrance Attendant unbeknown to me of course. Well as you know yourself if your from the USA, Americans make a big fuss over visitors, especially back in 1982, plus they are very big on customer service and do treat you like your someone special, which of course we tourist lap up. For some reason my daughter passed by me, so I ended up the last one to go through, and the Attendant really started making a big fuss of me, hearing my english accent etc, the others just stopped and waited, but when I passed through my daughter had the biggest scowl on her face, not happy at all, so I asked what's wrong and she replied '' do you have to talk to everyone Mom, especially the younger ones'', both my husband and son burst out laughing and told her not to be so jealous. I felt awful about it, I was not flirting just answering his questions being polite, but of course I was smiling and my daughter got the right hump over it.
Well that came up in the conversation when we were talking a while back, and she admitted that yes she had been jealous, not only on that day but during the whole holiday for all the guys were looking at me. I never even noticed, because I was not a flirty type of person, but we did laugh about it many years later. I do recall at the time though that I did realize that perhaps still being a young Mom myself is not the best thing for your children.
For you I suggest you don't ask her not to do it, for she will, but when you go to your Dads, take all your best clothes with you, even if your just staying overnight. The rest make sure they are dirty, it's the only way to fight your young mother back. She should not be doing it to you though, it does rob you of your own identity. Sounds like she is wanting to revert back to her younger days, but she risks making herself look silly dressing in teenagers clothes. Maybe not now, but when she is 40 people will laugh at her if she dresses too young.