Realistically? I think the mirror universe would be unsustainable. "The Enemy Within" is also a dramatic conceit, yet is a metaphor for the human psyche. That the mirror universe would be "just like" our own, only evil, is ridiculous.
Common political labels, such as party names or even descriptors (liberal, conservative) turn over so rapidly that they serve little purpose, except to get people riled up. I prefer terms like "creator" and "destroyer," although they seem a bit colorful. The mirror Starfleet are depicted as destroyers. They might last for a time, riding on the backs of a civilization that had been created. But eventually they would consume everything, and the system would collapse. (The Nazis would be the closest analogy.) In the climactic transporter room scene, Kirk tells "evil" Spock that it can't last:
KIRK: The inevitable outcome?
SPOCK: The Empire shall be overthrown, of course.
KIRK: The illogic of waste, Mister Spock. The waste of lives, potential, resources, time. I submit to you that your Empire is illogical because it cannot endure. I submit that you are illogical to be a willing part of it.
Unless one happens to be a collaborator, civilian life is probably short and miserable, and everyone has his own, personalized agonizer and booth.