Mirror Universe is a Star Trek trope, not essential to the series. Note that TNG and Voyager didn’t do them.
Discovery doing a mirror Universe episode is out of place this early in the series.
However, VOY and TNG played with the trope in Yesterday's Enterprise and Living Witness.
In all honesty, I'm always down for a mirror universe..any mirror universe. The trope in ANY scifi series is fun for me, just as recently as loving Agents of Hydra in AoS. But now that Discovery is in full motion, I do kind of feel like it would derail the story as of yet, but we'll see I guess. I suppose it all boils down to how they do it (if they even do).