I don't think things are worse now than they were before. The sexualisation of women (sometimes to the point of misogyny) has always been around in pop music.
In the noughties we had the Pussycat Dolls marketing themselves as modern burlesque; many singers and pop groups posed for FHM; we had TaTu pretending to be fake teen Russian lesbians in school uniforms, Britney, Madonna and Christina Aguilera kissing on stage, Christina performing in leather chaps and small knickers; Kylie relaunched her career purely with her arse...
In the nineties, we had the advent of Britney (16 years ago!), doing more of the titillating school uniform thing; Janet Jackson posed for an album cover topless but for a man posing her breasts, and Madonna released a book called "Sex" and had the whole erotica era. The Spice Girls were around, but Geri started out as a topless model to try to get noticed. The biggest girl band of the 90s was TLC and they were very sexual, not so much in clothes but definitely in content. Salt N Pepa were around with songs like "Push it" and "let's talk about sex".
In the 80s, Sam Fox released a (quite excellent, but that is by the by) record. Sabrina released "Boys Boys Boys", and in the video she is bouncing around in a swimming pool and her top falls so that you can actually see her nipples.
In the 70s, Donna Summer made a hit out of making orgasm sounds. Sometimes the women weren't even the singers but still their bodies were used to sell records - Pan's People anyone? (I've posted a couple of very unwholesome videos of theirs on another thread). Many a 1970s compilation record would have an inexplicable photo of a pretty girl, sometimes scantily clad or topless.
In the 60s (now this is niche, but bear with me, I know more about French pop from this decade than Anglophone) you had Serge Gainsbourg who used A.N. Other woman for his hypersexual songs- Je T'aime...Moi Non Plus, more orgasmic sounds. He wrote hugely suggestive songs for very young, wholesome, looking women like France Gall (there was one about sucking lollies, when she performed people in phallic lollipop costumes danced around her). She later commented that she was mortified by this, as she had no idea that it was suggestive, and she felt betrayed by the adults around her- to be honest I am more unsettled by this kind of humiliation of a woman than Nicki Minaj consciously getting her arse out and getting paid well for it as she has the rights to the songs she is performing. The 60s also had some hugely horrible behind the scenes exploitation - Tina Turner's horrendous abuse at the hands of Ike is well known. Tammi Terrell and all number of Mowtown singers suffered similar horrors. Berry Gordy ran Mowtown and was hugely controlling of his artists, so it was no better back in the good old days.
I guess the question is are the popstars of today being exploited? Are they being subversive and provocative? Are they being artistic and expressing themselves? How autonomous are they? I think there is a bit of a problem with the representation of women in pop, as there is in all parts of our culture, but the problem is an age-old one and I worry that saying "it's so awful nowadays, it was so much better when I was young", when it wasn't much better, devalues the argument a bit.
I also think it's not at all true that the music business and consumers only seems interested in overtly sexualised pop stars. There are countless examples of sexualised ones, yes, but there are countless examples of non-sexualised ones who are having their own version of success - sometimes equally or even significantly more successful than Rihanna or Miley. It was always like this - Janet and Madonna flashed their boobs in the early 90s, Cathy Dennis and Dina Carrol didn't but still had huge success - Cathy Dennis carried on writing and penned some of the biggest hits of the last 20 years. Jesse J, her clothes are a bit revealing but she started out writing for people like Miley Cyrus, and continues to write, so her career is not just because she shows some skin.
I think it's a case that we only remember the shocking things, which means they are doing what they set out to do. It doesn't mean that everything is shocking, or a female pop artist has to shock and push the boundaries of taste and decency to be successful.