I never get nice clothes, and all the ones i have are scruffy/unflattering!?

im 13, and all my friends look wonderful in all of their clothes! im so jealous of them, and i know that's not healthy, but i can't help it. i've asked my mum too take me shopping etc. but she never agrees, and just says mine are fine! when they really aren't i think i look ugly in all my clothes, and i feel as if other people will think this as well and that's just.. annoying, any suggestions on what too do?x