Is putting the computer to sleep while it's in a hot car a bad idea?
Putting a computer in a hot car is a bad idea. The additional heat, generated by a sleeping MacBook is almost zero.

If you have to leave your computer in the car, and temperatures are (or will reach) near the storage limits (45°C/113°F) you should leave it in an thicker or insulated bag, that will protect the computer from the heat. Avoid direct sunlight at all cost.