Nicki gets a lot of hate for her music, but she's actually really good. She has her fair share of stupid songs, and she's not really leading the battle when it comes to any form of social justice, but she's a good rapper. Her verse on Kanye's Monster was so good that Kanye almost left it off the track because he thought it made the rest of them look bad.
It's also amazing that she has risen to this level of fame as a female in the rap/hip hop industry. Rappers like Kendrick and Macklemore aside, the rap industry is incredibly sexist. For a woman to be able to shirk the object status and become an actual respected rap artist is an amazing feat. What other women are on Nicki's level? Lil Kim? Sure, she's ridden the BOOTY<3 train a little bit, but she does have talent.
All that said, she's no role model for body image. She's the gold standard of the "IF SHE DON'T GOT BOOTY SHE'S NOT A WOMAN" bullshit.
Taylor is a devil in different clothing. She has literally turned the term "America's Sweetheart" into cash. As much as her music and persona portray her as a sweet little angel that's blossoming into an artist-loving picture of feminism, she's a businesswoman. She's not worried about the little artists or the girl who hates her body. She's worried about her payday. She's ridden the slim white girl with makeup train just as far as Nicki has ridden the BOOTY<3 train, and she's riding it for money.
All this to say that they're both talented, they're both powerful women, but neither of them can say shit about positive body image.