Many a great entertainer has a moderately good voice. Johnny Cash didn't have a great voice, but wow, could he tell a story with a song. Likewise, there are many that have great voices but would be terrible on stage, so you never hear them. TS appeals to the young teenaged girl demographic, it's what she does.I like her, she is entertaining and there are a lot worse role models for young fans.
But in terms of all time great female vocalists I have seen in my lifetime, I would not put Taylor Swift in my top 10.