In the mirror world, an environmental or genetic factor damages the third type of photoreceptor present in people's eyes, which are little known/understood even today in this world.
New research shows that in addition to rods and cones, the eye has a third type of photoreceptor which is connected to mood. When these detect less light, e.g. because it's winter and days are shorter, the result can be a more dark mood.
If the denizens of the mirror world all took a journey to that world similar to your hero's, something about that journey could have damaged these photoreceptors. If they started out there, something about the common environment in the mirror world could be what causes the damage.
For example, a common environmental cause might be a particularly dazzling flash of light, or longer-term exposure to a particular wavelength of light we don't consciously register. That light could have come in a flash, like from the detonation of a nuclear or electromagnetic pulse weapon (or a flash on the journey to this world). It could have come from a solar storm that hit the planet. It could have come from a popular technology, like CRT or plasma or OLED televisions or early-generation smartphone screens, etc.
It's not at all hard to imagine that people adopt technology when the effects of that technology are incompletely known and unknowingly damage something they don't understand about how their bodies work, with delayed follow-on effects that are hard to definitively connect back to the actual source. Depending on how bad/smart <PERSON> is, he could be the one who developed that technology or promoted its adoption, fully knowing about the effect. To revert this at the end of the story, you could try an "enlightened population" route where people know about the effects and stop using the tech (e.g. Thalidomide) or have a newer better tech come out that by coincidence doesn't produce the ill effect (e.g. cathode ray tube displays, with their high pitched buzz), and permit natural healing. 209
The enlightenment option could come as a result of defeating <PERSON> and uncovering his documentation about the effect and plans for maximizing it. Or, have <PERSON> defeated as an almost coincidental side effect of the new tech disruption. Because he's pushing his tech for other purposes, he'll refuse to adapt and double down on a failing investment, pushing himself into bankruptcy and damaging/destroying at least some of his own capacities for other evils.
The issue could also come from a virus or illness that is highly contagious (e.g. pink-eye) but otherwise relatively asymptomatic (or leaves sufferers with a very low level of chronic pain, like a dull headache), and because it's extremely common it's considered normal, so the negative consequences are underappreciated and there's very little research done into causes or fixes, as with many other common issues. To revert this at the end of the story, develop a cure.
If you want the Big Bad Evil Guy to have agency, CRISPR and similar gene editing techniques could have been used to release a virus or bacteria with the explicit intent of spreading dark moods and making the world more evil. He might have taken a lesson/template from Toxoplasma gondii, which messes with host animals' brains to promote self-destructive behavior that can help get the parasite into its preferred host (cats).
It could be a genetic change, or an epigenetic change, passed on from one generation to another. That one's harder to overcome at the end of the story, but some tech could be developed to help people see better, just like we can currently develop tech to let people see in normally invisible parts of the electromagnetic spectrum.
Whatever the issue is could affect rods and cones too, to make things more gray and less colorful, but that's not strictly necessary for the main effect.
To the extent that it's understood, you could even put this story in the present world. Maybe the main difference between our current society and the "Garden of Eden" was a genetic change in our visual system that prevents us from seeing the force of love/God or the existence of abundance, permitting scarcity narratives that divide and cause people to do all sorts of nasty things to each other.
Any sufficiently advanced technology is indistinguishable from magic, and advanced technology is (often) based on advanced understanding. There's a lot we don't understand (some of which, we are just starting to understand) about the forces that govern our universe and how we as humans interact with them. Touching on some of these themes could make the story more interesting, depending on the overall goals and ideas you have for your story. Good luck! 693