It's a feminist song because it's about women being in control of and openly expressing their sexuality in a way that's not about the male gaze.

I’m going to explain why I don’t think Anaconda is a feminist song (although I do want to say that I fully respect Nicki as a feminist. Just not the song) : 

She plays exactly into the male gaze. Everything in that video was made to sell sex. Her body, the bodies of the dancers around her, the way they’ve moving. Nicki may have directed the movement, she may have had complete autonomy over her body and the entire video, but as a businesswoman, she knows what she’s doing and what she’s doing is selling herself. And she is commodifying sex. This isn’t my saying that she can’t do or dress however the hell she wants, but she’s also dressed how men want to see her. 
Secondly, the line “fuck those skinny bitches” assumes that Nicki is making space for women of all sizes. She’s not doing that, not at all. I didn’t see the issue with this until I was talking to one of my closest friends who is Somalian and was saying that black women who are thin and who don’t meet that cultures ideal of beauty (big ass, big breasts, small waist) aren’t looked on kindly. Nicki isn’t talking about fat women when she says fuck those skinny bitches. She’s talking about women who look exactly like her and the dancers around her. She’s talking about an unattainable beauty. 
Someone said that the video plays into the gaze of the queer woman, and this may be true but honestly, it is pretty much universally acknowledged that every man’s fantasy is watching two women get it on. That’s what’s happening in the video. She’s doing what men want to see. Again. 
I do agree that there are moments when she subverts this. Things like chopping up and throwing away phallic shaped objects but I sort of think that the rest of the video completely undermines this. She’s fucking wearing a maid’s costume for god’s sake. She’s squirting cream all over herself. It’s not a feminist statement in my opinion, not in any sense of the word. 

and that’s my pinch of salt on Anaconda.