Rihanna is a very "different" performer which isn't universally appealing. She isn't the objectively "great" performer in the same vain as someone like Beyonce, but she has her own style and sticks with it.
Personally, I love her performance style and the way she presents herself on stage. Yes she can be lazy as hell and it's super annoying when she doesn't sing her songs, but whatever.
I still love her and she's still winning so if she's good, I'm good.