Offensive and defensive technology tends to follow each other closely so there is nothing to say your mirror armour has to deflect all the energy to be effective, just enough so the surface and some fancy futuristic super conducting material underneath can vent the remaining heat energy else where. 

(if someone could invent a 100% reflective surface without impurities they most likely could also design it to reflect the laser back to the firer making the attacking weapon unsafe so possibly there is another reason why you wouldn't find lasers and mirrored armour on a future battlefield)