Sometimes these get stripped when someone tries to screw the right side mirror into the left side or the left side mirror into the right side.
Yamaha does righty-tighty on the left side and lefty-tighty on the right side. Sort of like old Dodges that had right side and left side lug nuts.
On your bike, one option may be to use a thread restorer-------the kits are sold at Sears and other hardware and tool stores, or your local mechanic should have a set.
Not necessarily cheap if you will use only one time, but I have a set in my toolbox for both standard and metric and they were definitely a good investment if you want a complete toolbox.