I think the right thing would be honest with your mom, but also be understanding if she chooses not to attend. Really, I think it would be very difficult for any serious JW to attend a Wiccan wedding, given the JW beliefs. But that could be said of many devote Baptists, Catholics, etc.
If you really want a Wiccan wedding, then all means...have one! If your mom feels she cannot go, respect her wishes. Let her know you are sorry she cannot attend, but leave it at that. I think it would be worse NOT to invite her. If she says, "if I had known it was Wiccan, I wouldn't have wanted to be invited" then just calmly say: "Mom, I want you to be a part of my life no matter what religion you are. It is your choice, I respect that, but I don't want you to think I was snubbing you, and you should know."
Man, I am wordy tonight! Good luck!
If your mom attends, then good for her! If she doesn't, chalk it up to the dub in her...not that she doesn't love you.