A lot of people don't really seem to take Nicki Minaj seriously, which is sort of understandable from one point of view. Her outfits can be silly, everyone saw Anaconda, and there was that song where she said "You a stupid hoe" like 40 times in a row.
But get past the canonical "bad songs", and she delivers a lot more than most other artists who've been out in the last 5-10 years. She has a wide variety of content, good range of flow, a variety of tempos and is a very strong feature.
In that sense she reminds me of Eminem in that she usually takes over songs that she is featured on and dominates them. On top of that she's one of a very few rappers who've done a song with Eminem and made it work without being relegated to background noise/chorus, Royce de 5'9" being the other that immediately comes to mind.
Not saying she's the best by any stretch, but it's difficult to name more than a handful of mainstream rappers from recent times who are for sure ahead of her. Add on that the talent pool for female rappers is sadly not very deep and I think she should get a lot more respect than she does.