I asked my father this question since he has a Ph.D. in physics and also a solar panel on his house. Here is what he said,
It would help, but the economics might not justify it. Probably less than 25% of heat enters houses through the roof. The reflectivity of the mirrors would probably degrade to 60% or less as they age and get dirty. It makes a lot more sense to use that space for water heaters or solar electric panels. On a sunny day, my roof panels provide enough power to run both the A/C and the pool pump, which are my biggest power eaters. When one or both are off, my meter runs backwards. The pool heater panels had the pool temp up to 90 degrees last week. I had to cut back on the pumping time.