[Puppet]

Can someone explain to me why a simple mirror isn't (or maybe it is) an effective countermeasure to a laser?

[MindBender26]

No mirror is 100% reflective. In combat environment, 80% would be about the best you could hope for.

So, since laws of physics state: "energy is neither made or destroyed, only converted to other form" then mirror would absorb remaining 20% of energy, convert it into heat and mirror would quickly shatter from ezxcessive heat.