FYI: I'm IS.
Aw stereotypes, got to love them. I've known GG's who have never acted "girly". Personally I've acted kinda girly most of my life, but I was kinda oblivious. Besides, I'm more of a tomboy.
Anyways, just tell your mom that not everyone who comes out as trans, has acted "girly". Besides, how would she know if you would be uncomfortable in girls clothes. Boys clothes are so, blah.
Best of luck!