Pip, I have never found any historical evidence that Mulatto was meant to categorize all non-white people. Indeed, in John Bouvier's law dictionary, published in the 1850s:
"... properly a mulatto is a person one of whose parents is wholly black and the other wholly white; but the word does not always, though perhaps it does generally, require so exactly even a mixture of blood, nor is its significance alike in all the states.)"
From the Medway v. Natick, 1810: "It is our unanimous opinion that a mulatto is a person begotten between a white and a black. This is the definition given by the best lexicographers, and we believe it also to agree with the popular use of the term."
I have seen no evidence that 19th century Americans considered Indians, or mixed Indian/whites, to be mulattos. It seems clear that a certain degree of African blood was necessary to be a mulatto. Indeed, I just found a newspaper that mentioned a lady as a "quarteroon Indian", not a Mulatto. I can believe that more modern-day people, anxious to downplay any African ancestry, try to pretend that their ancestors were "really" Native American or what-have-you.