The basic idea is good but imagine how many mirrors it would need to heat up a planet, and how would you keep them in a stable position, pointed in a set direction?
It's easier to blast a hole in the side of a cliff and seal it in. With a little insulation it would need little heating.
I would guess that nanotechnology could easily create an adequate supply of mirrors, and possibly they would not all need to reflect solar rays all the time, to provide correct levels of terraforming benefits.