Your mom really cares for you, I think you should just tell her this in a nice manor: "Mom, I really don't want to go. I know you love me and care for me deeply and truly; however it is my choice and I really do not like how you are constantly bugging me about going to homecoming. I've told you a lot that I just don't want to go, can we please just leave it at that?". I think she'll understand.