Just gonna say, the reason it's racist is because (as you said) it's derogatory, and it's based solely on a racial connection.
Hypothetically, let's say instead he called her "O'Leary", as a reference to the football coach George O'Leary who lied on his resume about his college education, and subsequently lost a job he had been given at Notre Dame. In that situation, he would be using "O'Leary" as a derogatory term, but it would not be anti-Irish racism, because the connection was based on a comparison of Warren (presumably) lying about her heritage with O'Leary lying about his education.
In this case, the only connection Warren has to Pocahontas is Native-American heritage, and the only reason Pocahontas is being referenced is because Trump is calling Warren's heritage into question, so it is clear that he is invoking Pocahontas' name solely as a race-based pejorative.