The structure of Trump's insult was that Warren was a faux-Indian, and the use of "Pocahontas" was a way of taunting her with the image of what she believed she was, and perhaps hoped to think of herself as, but is not. "Pocahontas", therefore, is being used as a synonym for "female Indian"; a family-friendly equivalent to "squaw".
Now, there may be an angle in which "Pocahontas" is used to indicate not female Indians, but the a romantic Western stereotype of female Indians, with the implication that this is the sort of flattened stereotype that Warren, like many Americans with a "Cherokee princess" somewhere down the family tree, may be plausibly accused of buying into. But Trump, like his supporters, is a barely-literate philistine, he is simply not operating on that level of cultural nuance.