Yes she is mocking her. She is mocking her as a rapper. It's different from blackface. No I am not excusing actual racism, as this is not racism. Jenna marbles isn't burning crosses and making blacks sit at the back of the bus. She will never do that. She is the least of black peoples worries.Jenna Marbles parodying Nicki Minaj IS racist. There is no satire in that, she is not making any statement about racism or blackface. If you dress up as a Black person to mock them, that is racist. That is actual racism. You are excusing actual racism.
The difference between RDJ's blackface and Jenna Marble's blackface is that (as far as I can tell) RDJ's is intended to be a critique on Hollywood racism, Jenna Marbles' blackface is just used to mock and caricature Nicki Minaj, which is racist. Jenna Marbles has also done other racist things on YouTube as well, she's made derogatory comments mocking an Asian man. You can look this up. This is blatant racism, and it is indefensible.
Now Jenna Marbles has gone on record publicly apologizing for this, and obviously her racist actions in the past do not appear to represent who she is now so I forgive her. However, what you're doing is you're denying that the racist things she did were racist, and that's the problem. What Jenna Marbles did was actual racism no matter how you spin it.
The asian thing is iffy. But then there is this. People make asian jokes all the time. Where is the line that's crossed thats makes it considered racist? A joke compared to actual racism like putting Asians in internment camps during World War 2.