First, I want to make it clear that I am not on your side this time. I always try to put myself in your shoes before making judgments but not this time. Sometimes you take my advice and it pleases me, other times you choose not to. I hope you do this time.
Mum has told you so many times that she doesn’t want you to wear make up or mini-skirts. She might be old-fashioned - as you pointed out but I think you should hear her out.
It is natural for girls to want to look beautiful and attractive but you will have all the time for that when you grow up. You are just a teenager for crying out loud.
Older women wear makeup to hide their imperfections and aging. Young girls like you should embrace your natural beauty and innocence. Personally, it saddens me to see girls in secondary school wearing makeup. It looks like they are being forced to grow up.
You said that dressing decently won’t make you feel beautiful and that people won’t like you because of the way you dress. That is not true. Dressing decently shows how much you respect your body and it is less likely that someone will view you as an object of attraction.
It is possible to be beautiful and decent at the same time. Just listen to mum because she loves you unconditionally and wants what’s best for you.