Not exactly. The term "Terran" was not used in "Mirror, Mirror." It was used in DS9 by nonhuman characters to refer to humans, in the same way that it's long been used in science fiction as a futuristic or alien way to refer to Earth people, or as a less hokey-sounding alternative to "Earthling" (and indeed Star Trek has frequently used it that way in non-Mirror episodes). So the DS9 usage was meant to be aliens' term for humans, not humans' term for themselves. The first use of the term "Terran Empire" was in the Shatner/Reeves-Stevens Mirror Universe novel trilogy, and it was first used onscreen in ENT: "In a Mirror, Darkly." ENT and DSC are the first series to use "Terran" exclusively for Mirror humans, and that's the problem. It's not about whether the Mirror humans refer to themselves that way; it's about the mistaken assumption that Prime humans do not refer to themselves that way. Since all of the first five TV series (counting TAS) had instances of characters using "Terran" to refer to people from Prime Earth, the Federation's Earth, it therefore doesn't make sense to have Prime-universe characters saying "Terran" as if it referred uniquely to Mirror humans.