It depends on the mirror. If it won't shatter from the sudden heat and expanding air, the electricity will form a Lichtenberg figure - Wikipedia. See My mirror a year after it was struck by lightning • r/pics. read more
The earth have its own resistance so when the lightning strikes to the land the change(current) is flow in the land due to the resistance of the earth. read more
If you're standing around outside with a mirror in your hand during a lightning storm you are being stupid and you deserve to be struck by lightning, and guaranteed, wherever lightning strikes, it will happen regardless of whether you have a mirror in your hand or not. read more
If a bolt of lightning were to strike a mirror, it would simply blast through the mirror. Now lightning is very bright, emitting a lot of light. Mirrors can reflect this light, if it happens to shine on the mirror, with ease. read more