The Mirror Universe is one of those tropes that allows the show's writers to do pretty much anything. We've had everything from time travel and resurrection (I mean, yeah, it's a Mirror Georgiou, but basically she's back from the dead) to creepy alternate timelines (Burnham/Lorca—gross), cannibalism (sure, not technically, but c'mon), characters acting completely unlike themselves, fungus heaven, and extremely weird haircuts. It's kind of a mess.
When you've got a plot device that can make almost anything happen, it seems like your story should explode with new and interesting possibilities. But most of the time, and certainly in STISCO, the Mirror Universe has the opposite effect. That's because fascinating stories are built from equal parts structure and free play. Especially when you're in the science fiction or fantasy genres, a big part of the fun is figuring out how the rules of the world work. There's a distinct pleasure in worldbuilding that sets up limits, because that's when our heroes come up against complex challenges.
The rules of a world can be outlandish, of course. You can have a spore drive that blips all over the universe and accesses a "mycelial network" that's packed with macro tardigrades. That's fine. Problems start cropping up when literally every episode invents a new magical thing that the spore drive can do. Yep, it can travel to alternate universes. Yep, it can travel through time. Yep, it can access the spirits of the dead. At a certain point, this stops being amazing and gets, well, boring.
As I watched the last few episodes of STISCO, I realized that I'd stopped caring what happened because there were literally no stakes. People could basically come back from the dead via their Mirror versions or if the ship traveled back in time (I dread the possibility that we'll see this subplot in a future episode). Nobody had to worry about running out of spores, because Stamets could just destroy the ecosystem of a handy moon by injecting fungus colonies into its crust.
Of course, Star Trek has always had its share of magical tech and just-in-time transporter saves. But, generally, the franchise doesn't suddenly do a 180 and turn transporters into time travel devices or make replicators capable of churning out sentient goblin armies. In Voyager, for instance, one of the major technological limitations (other than the speed of the ship) was that the holographic doctor could not leave the medical bay. But over several seasons, this situation changed. First, the Doctor had a longstanding wish to move around freely. Then he got a piece of future tech that allowed him to wander the whole ship.
The fact that this change merited an entire episode of discussion—and was part of a longer character arc for the Doctor—illustrates how limitations lead to creative challenges. How boring would it have been if the Doctor had suddenly said, "Oh, I just discovered that I actually can move around the whole ship"? There would have been no struggle and no chance for us as viewers to develop sympathy for him as he worked toward his goal.
I'm not saying the characters on STISCO don't struggle. But the rules change too fast for us to feel like it takes heroic (or even ordinary) effort to overcome challenges.
Then came the reveal that so many of us had glumly predicted: Lorca is actually from the Mirror Universe. Suddenly, the murky ethical landscape of the Discovery became a simplistic black-and-white. Lorca wasn't morally conflicted; he was just evil. Stamets wasn't going crazy; he was just in touch with the Mirror Universe.
On a macro level, the Mirror Universe also had the effect of letting the Federation off the hook for all its problematic decisions. In the first half of the season, we felt a sting when Discovery's scientists were told to turn their research into weapons. We experienced Burnham's shame over starting a war that was avoidable. Saru had to swallow the Federation's orders to kill his sparkle people friends to improve the fleet's signals intelligence. Sarek, who was shaping Federation policy, nevertheless felt that humans were an inferior race.
My point is that the Federation was doing all kinds of things that would have gotten Picard's knickers in a bunch. Our beloved interspecies institution of exploration began to seem proto-fascist, at least until we saw the so-extreme-it-felt-like-parody fascism of the Mirror Universe's Terran Empire. Once we've seen Burnham eat the brain tentacles of Saru's pals while everybody on the Mirror Shenzhou murders each other to get promotions, the Federation can claim the moral high ground.