It is just my opinion, but I think that if your mother is bringing it up lightly/jokingly, she will probably be accepting if you tell her. In fact if she is joking then she has probably already started to entertain the idea. Just speak from your heart and let her know how you're feeling. She might not fully understand it at first, but then again I don't think you can expect anyone to understand something so complicated and new the first time you tell them. But at least she has helped you to initiate the conversation, so it won't be as awkward as if you just brought it up one day "out of the blue."
Myself, I just sort of winged it. I was very nervous about telling my dad, because for him it would be a completely new concept to grasp, so basically I just started walking over to where he was and then put myself on the spot. It went much better than I planned, because he didn't deny what I was feeling. I don't think your mom will, either. She seems to really care about you.
(Also, if you do tell her and every once in a while she jokes about the whole thing, I wouldn't let it get to you. For some people humor is a coping mechanism, so they can get their head around the idea.)
Good luck, Brent!