Good post, and very good example.On the unlikely chance that I'm not missing something, I don't think calling somebody Pocahontas is intrinsically racist.
They're mocking Warren for claiming to be Native American by comparing her to somebody who definitely is. It'd be like calling a stupid person "Einstein" sarcastically; it's not an insult to Einstein or smart people in general—it only works because the stupid person fails to match him in comparison.
To clarify, I have no problem with Senator Warren, and I think the comments are stupid and childish. Not racist.
As others have pointed out, I think people are so quick to label statements as racism. I personally think that the Pocahontas comments are just dumb, but not racist.