We owe it all (or at least mostly) to Lady Gaga, so I'll start with her.
Lady Gaga

I think her "style" is pretty obvious..
1- random oversize object on face to hide colossal nose --- check.
2- dress thing fabricated with Hefty bags that manage to cover everything EXCEPT crotch/boobs... check
3- Shoes that are so insanely uncomfortable and hideous that no one would ever actually wear them... check
I know she calls it fashion but lets just be honest... WHO WOULD WEAR THAT? I'm pretty sure I've seen homeless people dress better than that... actually I don't think I've ever seen a homeless person who didn't look better than that. Please GaGa, stop trying to build up your image. We get it... You want people to think you're a transgendered anorexic cave troll.
Rihanna

I'm not sure if she gave in to pressures to be more provocative to draw attention to her music or if we have Chris Brown to thank for this one.
Have you seen the video for "Rude Boy?" With lyrics like "I like the way you touch me there. I like the way you pull my hair." I was expecting sexy lingerie and stripping. Instead I got Rihanna in a hideous body suit humping a zebra covered in balloons. It was like Andy Warhol meets Bob Marley meets an acid trip. Yummy.
Christina Aguilera

Now I can't be too hard on Christina since her career is arguably over. This is clearly an attempt to revive it. I just don't get what diamond studded gags/chains and freaky eye makeup does to help your career? If someone would explain this to me that would be awesome.
Beyonce

This one pains me. I love Beyonce. She's confident, sexy, sassy... pretty much everything I'm not. Her albums are among the top selling of all time, and every song she's released in the past 5 years has been number 1 on the charts at some point or another. I just don't think she needs to dress up like C-3PO to get our attention.
Wrap Up
When did the concept of a pop star become less about selling sex and more about dumpster-diver fashion? Please don't think I'm complaining about seeing less T&A. I just think that this shouldn't be the next logical evolution of pop music. If these divas continue to out freak each other will there really be any shock value left? I can't imagine this trend can continue much longer before the consumer gets bored.