all you have to do is get a gold medal in all the other races of all the other cups, then you should unlock mirror.
To get mirror cups in Mariokart Wii you willk have to complete all of the cups on all engune sizes
there is no hat cup
Beat the star cup
The Special cup
You unlock it by getting gold trophies in every cup in 150cc. And you don't turn mirror on and off; it's a separate difficulty altogether.
By finishing the race's
by getting gold on the star cup
50cc 100cc 150cc mirror mode.
The Shell Cup, Banana Cup, Leaf Cup, and Lightning Cup.
Get a Bronze, Silver or Gold Cup in the Shell Cup and Banana Cup.
Mariokart wii
Get the Gold cup in all 50cc , 100cc and 150cc cups