There is some debate about it. But it not really about that. So, for sake of discussion, let's say she did.
Now, you stated it correctly, and if Trump had said that, no controversy. But he instead chose to use a well known and respected member of another culture to disparage someone who falsely claimed to be of that culture. Suppose a Caucasian portrayed himself as African American, to get something for himself. Would it be appropriate to start calling him Martin Luther King, or the name of some other prominent African American? I would label it "highly inappropriate". I realize Trump was disparaging Warren, not Pocahontas or her ethnic culture, but WHY "Pocahontas"?