Mirror mode is actually pretty trivial to add. You just do a post-processing shader that flips the X-axis. Then, you flip the controls manually. Anyone with basic shader knowledge could offer an unofficial 3rd party solution (like we had with a few custom shaders for Dolphin in the past). I mean, if someone wanted to, they could try to add the shader Dolphin with a pull request of their own. Admittedly, some of the existing shaders are just as silly or "pointless" depending on your point of view. Only downside is backwards text, but not an issue for some games that mainly use HUDs and nothing else.
Anyway, there is some merit to wanting a "reverse mode". It triggers a sense of unfamiliarity in our brains when doing platformers right to left, also most people (mainly right-handed?) unconsciously associate that directional movement with fear. So it's a small way to spice up gaming for limited titles. Other stuff like Super Mario Sunshine, not so much, since there's free movement, and the text is all backwards in dialog-heavy games.
Anyway, there is some merit to wanting a "reverse mode". It triggers a sense of unfamiliarity in our brains when doing platformers right to left, also most people (mainly right-handed?) unconsciously associate that directional movement with fear. So it's a small way to spice up gaming for limited titles. Other stuff like Super Mario Sunshine, not so much, since there's free movement, and the text is all backwards in dialog-heavy games.