My mom thinks I don't appreciate her hand-me-downs, what should I do?

Well first of all your mother thinks she doing a good thing and passing on your Grandmothers stuff to you. You on the 2nd hand don't appreciate it and your young and sometime in your life you will value these things. This is love and many people are proud to have the things that belonged to their grandma's and grandpa's. So tell your mom, you don't want anything anymore and since you don't appreciate it, hopefully she will stop giving it to you.