I believe that solar panels were an option on the Maybach 62S, and they would run the ventilation fan while you were parked so you wouldn't return to a hot car after going to the store.
Like everyone else has said - there just isn't enough area on the top surfaces of a car to do any noticeable charging.
If you were to theoretically have a perfect 400W PV panel on top of a car, and left in direct sunlight, it might be enough to run a medium sized peltier/TEC cooling unit to somewhat cool down the car while you leave it parked. Or a very small heat pump. Would definitely add a lot of extra cost in manufacturing and complexity.
Or just keep the car fan running and use the existing AC system (in ventilation mode, no compressor) to keep the car just as hot as outside (instead of much hotter). If you have some spare power maybe even run the AC when the key gets back in range.