There are basically two Mirror Universes. In the TOS era, these are the same, but sometime afterward is where they diverge.
There is the canon Mirror Universe, where Spock arose to lead the Empire, reformed it, and then it was conquered by a Klingon/Cardassian Alliance.
And then there is the Dark Mirror Universe, where either Spock didn't take Kirk's advice or was killed off before he could do too much, and the Terran Empire survives into the 24th century, maybe managing to conquer most of the galaxy and other powers by then.
Out of the entirety of a multiverse, there's no reason to say these two versions couldn't both exist, so I don't want to give the impression that this is to tear down either version. The question is, which of the two is the one you like the most.
There is the canon Mirror Universe, where Spock arose to lead the Empire, reformed it, and then it was conquered by a Klingon/Cardassian Alliance.
And then there is the Dark Mirror Universe, where either Spock didn't take Kirk's advice or was killed off before he could do too much, and the Terran Empire survives into the 24th century, maybe managing to conquer most of the galaxy and other powers by then.
Out of the entirety of a multiverse, there's no reason to say these two versions couldn't both exist, so I don't want to give the impression that this is to tear down either version. The question is, which of the two is the one you like the most.