Mirror mode is the highest difficulty unlock in Mario Kart World, and essentially flips all tracks horizontally, or mirrors them. The process is fairly tedious to accomplish but not difficult even for beginners.
This guide has all the requirements for unlocking mirror mode in Mario Kart World.
Grand Prix Cups
This step requires mere completion, not even a win or trophy, of all eight Grand Prix Cups in the 150cc class. The lower classes will not count towards this whatsoever. Be sure that the green checkmark is visible in the 150cc class for each race.
Knockout Tour Rallies
Once again, this requires nothing more than crossing the finish line in all eight Knockout Tour Rallies with 150cc vehicles. Avoid elimination to the best of your ability, and survive to the end to acquire your checkmark for each race.
Free Roam Collectibles
Scour the map for 10 P-Switches (blue ‘P’ icons), 10 Question Panels (glowing question marks), and 10 Peach Medallions. Make use of the tracker to find ones that you have not yet scooped up, if you are having trouble filling out the requirements.
Repeat The Special Cup
Race in the Special Cup again after completing the three previous steps, specifically at 150cc. Winning or placing is unnecessary, simply completing it will suffice to unlock the cutscene and mirror mode itself.
Free Roam Mirror Mode
After unlocking mirror mode for tracks, you can also unlock it for free roam. Go to Peach Stadium, and drive until you can take a left onto a winding route. Watch for a red roof and drive on top of it, making a beeline for Peach’s Castle. Search for Peach’s window and drive through it to trigger a cutscene and mirror mode for free roam.
This will not work until free roam is unlocked for tracks.
That is how to unlock mirror mode for both tracks and free roam in Mario Kart World.