It looks like you are just perhaps going to have to accept it with grace and know that you are not the only one who is going to be there doing something not of your choice because it is expected of you. Just have a good sense of humor about it, and don't take it too seriously, and I think you will do fine.
On the other hand, can you compromise and wear it half of the time and half of the time wear what you would like? Would she allow that? On the other hand too...she is probably just wanting to love you by making you this, and one day, you will laugh your self silly over it, I'm sure.