Would a laser simply burn off dust on the mirror's surface? And would a powerful laser be more likely to react to blemishes and imperfections in a mirror- if, for example, the mirror was scratched or pocked? Would it then reflect off of the 'good' parts of the mirror but burn into the flaws, possibly warping or destroying the mirror?
I would figure that in order to use a mirror as a 'defense' the mirror would have to completely cover the area you need to protect, otherwise you may be dead before the laser hits the place where you put your mirror... you don't know exactly where the thing is going to be aimed. To use the mirror to actually turn a laser against its operator, you'd have to to hit the plane carrying the laser, not neccessary the exact point where the laser is emitting, nor neccessarily at the same 'power', but at least with enough intesity to blind the aircraft and its crew. But you would also have to have the mirror (or mirrors) aimed at the plane before it fires, because there won't be time when it fires. And you'd have to have your own weaponry in case the plane turns out to be conventional and drawn to your highly polished reflective shell.
It might be easier just to build your own laser and hope you get to fire first.