I would ask yourself first if it really affects anyone in a serious way, because I think your mom should be able to wear whatever she pleases. But if you really want to change it, try turning her over to some styles in her age range.
I have an aunt who does the same thing..she's in her early 50's and her daughter is 18. My 50yr old aunt wears shorter shorts then her own teenager daughter..they r so short that her butt is almost hanging out. I'm 30 with 4 kids and I luv looking n dressing trendy and fun..BUT with taste. Maybe a good idea would be to go with her and help her pick clothes that would look good on her. You can still dress cute and young but with class :) good luck honey
Your mom can look youthful, without dressing like you. Sit down with her and tell her how you feel. Then you and her can go shopping and find something more her age. Please do not take her to some old lady store.