Carmella is such a weird case because there's really nothing wrong with her. She's improved in the ring (not by much, but she's at least competent now compared to when she was first called up), she's great on the mic, she knows how to play her character. In hindsight, giving her the briefcase was a mistake (it really should have been Becky), but I can see the logic in that the office probably saw her getting better and wanted to do something big to see if she could really get over. Unfortunately, there's just something about her that causes her not to connect with the crowd.
She just might be one of those people who always needs a manager or a partner to really stay over.