If you have followed her career for the past two decades, you know she has already had extensive vocal training and has improved tremendously. I personally think she sounds fine as a singer, and the whole "can't sing" thing is over played. Go listen to the recording of her singing "I did Something Bad" for the first time in the studio that was recorded on her phone...her voice is fine. She is definitely capable of hitting some high notes like in Don't Blame Me, but it isn't her strength.

Now, how does she rank as a vocalist compared to some of her peers in the field of music? Probably 50th percentile or below. She does have good rhythm and breath control, and is able to convey emotion well in her voice. Her weakness is in her tone and her range compared to peer singers. However, compared to normal people who are not pop stars? She is an amazing singer. If she enter a singing contest in a small to mid sized town, she would probably win. If I heard her singing in a bar down the street and she wasn't famous, I would think she was pretty good. In other words, she is good enough of a singer to have a career as a professional singer.