I've got an aunt called Tippy! It's her middle name, Tippins, also my middle name, from a family surname. I'll edit this out. I like to consider it a short form of Theophania, which is Tiffany.
I think it's a silly and fun nickname.
Thora is super fun and I'm glad to see evidence of its use; I feel it's too pagan to catch on in my area, so I'm waiting for Flora instead, but that doesn't seem to be happening either. Maybe in the next decade. You're UK-based, right?
Gosh maybe Tippy is going to catch on in time for me to have a daughter and I can name her Theophania and call her that... or I could just do that anyway...