That assumes it's a true 'mirror.' It's more of a divergent universe where the humans, instead of embracing peace and exploration, decided to forge their own empire instead. Really nothing in the universe really shows the races other than the humans as different. The Klingons and the Cardassians are still warmongering, for instance.Never quite "got" the Mirror Universe ethos.
If our heroes of the Federation have evil counterparts, then does that not mean, or should it not mean, that all the badass guys like Klingons, Romulans, Borg etc should in actual fact, be good guys or peaceful explorers in Mirrorland ?
Well, I would tend to "assume" that if something is called a "Mirror Universe" then it would be a mirror for all in said universe, would it not ?
If other races still show the same character traits in this and the mirror reality, then it cannot be in essence a "Mirror Universe" as such.
Just a pesky "Mirror Image existence for Terrans Universe"
Bit too convenient for a tasty filler story in a long season of episodes.