I’m sure you feel irritated when your best friend wants to change herself and create another image which doesn’t represent her, but I can assure you that it’s a common problem nowadays. I’ll give you some advice to deal with it.
The best thing to do is to discuss with her and tell her how you feel. You should make her understand that being popular is not one of the most important aims that we have to achieve in life. Moreover, you could speak to her in order to help her comprehend that she doesn’t need to change her style of clothes or the music she likes to make herself stand out.
Furthermore, if I were in your shoes, I would spend some time with her productively and do something together to cheer her up and remind her how enjoyable those past days you spent happily was. This might be a good idea to make her realize that she doesn’t have to be cool in order to be accepted by another group of people.
I hope everything works out for you.