Light weight panels tend to produce less power. The best ones are opaque and have a thick glass layer on both sides of the circuitry - so they're not super light. Also the glass would need to be strong enough to handle rocks/etc as well as provide structural support in a crash (even if it's just with the initial impact and then shatters similar to a car windscreen).

They could theoretically be light however in reality heavy panels might be a better choice.

But yeah I agree you're on the money with protecting the car. This could make a big difference to the usable life of the battery by keeping it closer to the optimum charge level especially with a normal daily suburban to city commute.