now that i think about it gaga did influence rih's look a bit...the whole "edgy over the top" look that gaga rocked during the fame was replicated by rihanna during rated r except with a darker spin
I'm not going to say that Rihanna wasn't doing big shoulders and crazy costumes before Gaga, but I have a feeling that even if she wasn't, Gaga would have still been doing Gaga. It's just the way fashion was headed and music was headed towards dance-pop as well. Rihanna wasn't the only one at the time to get a #1 from a dance-pop song.
Lady Gaga didn't invent anything. There were people doing things similar to her for years, noticed and unnoticed.
But after Lady Gaga's tremendous, quick rise to fame, she made it so that if you weren't doing what she was doing (crazy costumes, etc.) you probably weren't going to get noticed.
Enter every new pop girl being over-the-top and Nicki and Katy becoming cartoons.