So, this move is coded according to forums, and with z move, it should be like this "When a Pokémon is holding Flyinium Z and uses its Z-Power, Mirror Move turns into Z-Mirror Move and raises Attack two stages, in addition to its usual effect as above." 

I have tested the same on sd, and for example, my opponent uses let's say, Ice beam, and very next turn my pokemon uses z-mirror move, it should boost attack by 2 stages and use Subzero Slammer, which is ice type z-move, because it copied Ice beam with a z-crystal. 

But, I tried to test the same with a friend, and we tested the same with 3 moves-

1. Opponent's latios using draco meteor

2. Metagross using zen headbutt

3. Metagross using bullet punch

In all the 3 cases, mirror move just boosted attack but didn't hit the opponent. 

 

Kindly look into this.

 

Regards,

Animesh

Hello, @Animesh3011
 

Thank you for taking time reporting the issue to us. However, Mirror Move is currently not coded, so it has no effect in copying the nearest move targeted at it, that explained the Z-Mirror Move you tested could only raise the ATK stat by 2 stages and had no other effect. 
 

Thanks again for your report, and feel free to send us anything you consider buggy in the future. Have a good day.
 

Regards,
 

Shiningluck