I wouldn't see most of them. "Pop girls" are only good for radio and YouTube. The majority of them stink on stage, save Beyonce.
But to answer the question...the ones who immediately come to mind as not being worth the ticket and gas money are Rihanna, Britney, Katy, Kesha and Miley.
When I saw her she was flawless. She really knows how to hype up a crowd and maintain the energy. She knows how to put on a really fun and entertaining show, and her vocals aren't that bad either. I mean- she's no Mariah Carey but I've heard worse. I didn't go to see her expecting to be blown away by power ballads; I went to have fun and I didn't leave disappointed.