Taylor Swift is not signed under a Christian label, but she did go to a Christian school growing up. However, she has not yet claimed to be a Christian. This answer is undefined as far as any informed person will share.
Yes, she is not signed as Christian, but she graduated from Aaron Academy, a Christian school, and a lot of her songs mention God, or something along those lines. And in her song Christmas Must Be Something More leads us head-on that she believes in God, since I doubt she would sing something she did not believe in. She has stated in a few interviews that she is Roman Catholic. It's very alike Christianity, however there are some differences. God is the ultimate Judge, lets leave it up to Him. (: