Oh for goodness sake I was expecting to be something either outrageous, hideous or dead common!
Tabitha is a very lovely name - classic, pretty, not over-used, and Tabby would be a lovely NN - what on earth is the matter with you?!
Isaodora is lovely too, but she will get called Izzie and thereare thousands of those.
Minnie is really cute as a nickname although I agree as a proper name it could be problematic in case she turned out to be very large.
I think you are allowing your mother's naughty manipulative comments to knock your confidence over this.
If you like Minnie and you like Tabitha (in spite of what your mother thinks) then keep doing as you are doing. Loads of people are never actually known by there official name. If she feels Minnie is too cutesy and not sophisticated enough as she gets older she can choose to revert to Tabitha.
On the other hand, if you took 6 weeks to name her and she has only been Tabitha for 3 weeks and you always call her Minnie anyway then I'm sure it won't be that big a deal if you change it now.
But even if you do, I bet you still just all her Minnie. Once a NN sticks it's really hard to shift.