I think I experienced the same reaction as you did, but was interpreting it in a different way. When I saw Ripley on Broadway, her voice was starting to go and I spent the performance genuinely concerned for the health of the actress, which, in turn, took me out of the show at points. She was brilliant and that rawness did indeed contribute to her performance in the role of a woman on the edge, but I still spent more time thinking about Ripley herself, rather than Diana, than I have ever done in a theatre about the performer. I wondered if I was seeing Diana on the brink of losing control or Ripley.
With Mazzie, all the singing was effortless and I was able to focus on the character. I heard at the time that Mazzie and Daniely were playing the characters at a higher socio-economic level than Ripley and Spencer had done and I think the effortlessness that Mazzie displayed in the singing reflected that.