1. The car's surface is small relative to charging needs. Both have fundamental limits (insolation and aerodynamics) that aren't likely to change much.

2. The car's top is not generally flat or even a simple curved surface, so you'll probably be compromising the aerodynamics.

3. If the car is parked in a garage, or under a tree, it will not make power. If you're buying the panels, put them where they'll see the sun as much as possible (rooftop, shade awning for parking, etc.) would be better.

4. The solar panel will get junked when the car gets junked rather than lasting longer as it would in a stationary environment.

5. If the car's battery is full, the panel output can't be used, but see point 1, this will only happen if the car is parked in the sun w/o driving for a number of days, even if it was parked at 80%.