Okay. If you needed a short term to quickly distinguish between Prime and Mirror Universe Humans -- one that isn't a mouthful, one that feels like a natural outgrowth of colloquial rather than formal English -- what would you use?
I reject the premise of the question, because the difference between them is one of nationality, not species. They are Federation humans and Imperial humans.
The thing is, the term "Terran Empire" was never canonically used until Enterprise, and I consider that a mistake. When DS9 had the Alliance members refer to humans as "Terrans," it was evidently intended by the writers as a slur, a term that aliens used derogatorily. That's often how "Terran" has been used in science fiction, as a term aliens use for humans. And in this case it was inaccurate, since not all humans are from Earth, so that compounds its nature as a term used by outsiders who don't care about the distinction.
So it was a retcon, and IMO a misinterpretation, for ENT to start using it as the Imperial humans' term for themselves. And DSC has just compounded that error by treating it as a species name. It would've been preferable if they'd stressed the "Empire" part instead of the "Terran" part, because it's the "Empire" part that's the meaningful difference from Federation humans. So I'd prefer it if they'd been called Imperials instead of "Terrans," because Federation Earthlings are Terrans too.