Some reflections on DSC's sojourn in the Mirror Universe...
In TOS's original "Mirror, Mirror," what we saw was one starship crew (among many) from the familiar Trek universe encountering a comparable starship crew (still just one among many) in the Mirror Universe. The characters had counterparts, but they were all similarly situated. To the extent that anything about the MU was ever plausible, this was.
In DSC's journey to the MU, however, things weren't like that at all. Instead the writers upped the coincidences (and the stakes) by a level or ten.
Imagine that you're Michael Burnham. You've served under exactly two captains in your Starfleet career. Now you find yourself in the MU, and lo and behold! Your previous captain just happens to be Emperor of the entire Terran Empire, while your current one is her main rival for that position, leader of an internal insurrection. What's more, you yourself are so pivotally important that the main thing these two power-hungry rivals are willing to fight to the death over is your loyalty and affection. And even more, among your shipmates, your lover is (in his Klingon form) one of the inner circle leading the rebellion against the Empire, and your closest friend, despite being just a cadet back in reality, is now a ship's captain with a fearsome reputation. And at the end of the day, you and your ship not only have to get home, you have to save the entire multiverse in the process.
Put it all in perspective like that, and the whole MU experience sounds less like an alternate universe and a lot more like a psychotic break, accompanied by delusions of grandeur.
If we give the writers the benefit of the doubt, we can assume they didn't intentionally make Burnham and everyone and everything associated with her important out of all credible proportion. On the other hand, they didn't think to stop themselves from doing that, either.
In TOS's original "Mirror, Mirror," what we saw was one starship crew (among many) from the familiar Trek universe encountering a comparable starship crew (still just one among many) in the Mirror Universe. The characters had counterparts, but they were all similarly situated. To the extent that anything about the MU was ever plausible, this was.
In DSC's journey to the MU, however, things weren't like that at all. Instead the writers upped the coincidences (and the stakes) by a level or ten.
Imagine that you're Michael Burnham. You've served under exactly two captains in your Starfleet career. Now you find yourself in the MU, and lo and behold! Your previous captain just happens to be Emperor of the entire Terran Empire, while your current one is her main rival for that position, leader of an internal insurrection. What's more, you yourself are so pivotally important that the main thing these two power-hungry rivals are willing to fight to the death over is your loyalty and affection. And even more, among your shipmates, your lover is (in his Klingon form) one of the inner circle leading the rebellion against the Empire, and your closest friend, despite being just a cadet back in reality, is now a ship's captain with a fearsome reputation. And at the end of the day, you and your ship not only have to get home, you have to save the entire multiverse in the process.
Put it all in perspective like that, and the whole MU experience sounds less like an alternate universe and a lot more like a psychotic break, accompanied by delusions of grandeur.

If we give the writers the benefit of the doubt, we can assume they didn't intentionally make Burnham and everyone and everything associated with her important out of all credible proportion. On the other hand, they didn't think to stop themselves from doing that, either.