Well, I guess it depends how you define Mulatto.
From my untrained eye I suspect they are a race that mixed long, long ago and could be a combination of European, Asian, and Indian blood. The skintone is rather golden, hair is straight to wavy brown, and the facial features do not appear African. That is except for the ones living in Kwa-Zulu Natal and they have a head structure and features that are consistent with Zulus and likely have mixed to be technically categorized mulattoe.
Years ago there were some small pockets of groups in Namibia, Botswana, and Zim. I wonder if the Zimbabweans have run them all out or annilihated them since they never categorized them as part of their inner circle.
If there are linguistic studies on their native tongue, that seems to be the most accurate way that leads back to the origin of a race. I have always wondered where they came from and have never gotten a straight answer, only speculation.
The realities of "outsiders," whether that alienation was due to religion, race, or what have you, meant that there were clusters of isolates, who were multiracial, and usually outside the law. The border region of North Carolina and Virginia is an interesting, historical study in this regard. These people lived on the border, in remote areas, intentionally, so they could leave a problematic jurisdiction in short order. Another, somewhat related term, but one wrapped in much more controversy and freighted with further historical meaning, would be "Melungeon." Derived from the French, melange.