Some people really don't understand. I get it from my boyfriend from time to time. My parents' accept it with as much grace and patience as they can muster (which is a lot), and they get excited when they see my costume really come together. I was surprised how much my mom told her friends about Davy Jones. Normally I think it's a quiet subject around my household.
Perhaps what might be best is if you talk to her about going to a cosplay picnic, a small event with a relatively small amount of people in it, together. To make her more comfortable, it might be best if you went in normal dress. You could introduce her to us and get her to talk to us to see that we're not the fetish cosplayers who do, erm, things in costume and that we just do it for fun and to express love for a show or character. I'd be willing to talk to her as Davy Jones. I consider my life outside of cosplay to be exceedingly normal. Cosplay is just the spice that makes my life more challenging, creative, and fun.
If it wasn't for cosplay, after all, my grandma's gift of a sewing machine would have just gone to waste. I love my sewing machine so much :3 I sew normal stuff, too! Like... like pants!
EDIT: By the way, I don't think going behind your mom's back would help anything. She might be pissed if she found out.
