I have no opinion about whether the Warhol car looks good or not, but you guys that think the amount of time he spent on painting the car determines whether it is real art or not or any good, just don't understand the creative process.
Almost every true artist will tell you the best paintings and drawings are the spontaneous ones that are sketched quickly and in the moment. Anytime you try to do a "second draft" of a powerful, spontaneous sketch, it loses its immediacy and some of its power. That's just the way art works.
It's similar to other non-scientific endeavors - be it music, or basketball or whatever, when you do something in the moment, the results tend to be great. When you over think it... not so much.