Is it bad to paint ..
 

The bottom/underneath my Mazda? My uncle did it with latex paint .. As a "surprise" it will "help with rust". Just curious if it's bad or not I've been driving around for a few days and it smells after being ran for a bit. I guess the heat is heating up the paint?

I think latex paint is designed for house exterior and interior. If you open a can of latex paint it smells like ammonia. That is because at the factory they add ammonia to the latex so it won't cure. After you paint the wall the ammonia evaporates, the pH level drops and the rubber cures. I suppose this step is completed on your car. What happens now is that the latex rubber gets burnt from the hot exhaust.

What do you think Ammonia will do to metal? I think it will make it rust. Amazing what people will do to their cars.