Yes and No. Season 4 consisted of a bunch of mini arcs consisting of 2 & 3 parters. So jumping into one of those in part 2 or 3 left you blind. The Mirror one was a 2-parter.
Not that I'm complaining, because season 4 was the best one, especially after the monstrosity of season 3. I felt the show finally hit it's stride in that final season, at least until that finale ruined everything.
Not to go too OT, but "In a Mirror, Darkly" had some of the best vfx of the entire series, alongside the final 3 episodes of the Xindi arc. I always enjoy the large space battle scenes.
At least Braga finally acknowledged a few years ago that what he did with the finale was a mistake, after initially defending his decision back in 2005.
However, I do think Discovery missed a key opportunity here. They showed the USS Defiant's schematics, explained how it had been taken into the mirror universe and could hold the information to get them back, but the writers never expanded on it. In fact, they returned to their own universe without so much as a glimpse of the Defiant itself. *facepalm*