No, you can't generalise..But specifically, Mulatto has always meant some part African blooded...Originally, it meant half Black...Later, people with less than half Black blood were also called Mulatto..but they still had Black blood..
As far as census takers go, we can speculate that they only went by what people *looked like* to their eyes, but odds are that long ago, census takers knew who they were counting..Either way, there is nothing to prove that they were wrong in classifying someone
Negro, half Negro (Mulatto)or some part Negro(Mulatto).
I have never seen a census with Octaroon on it yet..even though that was a term used for
who otherwise might be called Bright Mulatto...Those folks looked white..but they weren't.