I agree that Steve's family and friends treat him as if he were stupid, but I don't see that he's being presented as stupid at all. I know he acts the buffoon but he's pretty savvy underneath. People tend to live up (or down) to how they're treated, and I think Steve's condition has been exacerbated greatly by being constantly undermined by those around him.
This obsession with the car is about him escaping from reality - something Lloyd even said to help reinforce it to viewers. When Steve's thinking about the car he doesn't need to think about his own life, which he sees as a total failure.
I agree there's something amiss if many viewers aren't recognising the significance of his behaviour, but I've seen many positive comments elsewhere on the internet too. In fact, I'd say the positive comments outweigh the negative.
As for how far you'd have to go to see Steve not acting the buffoon, I'd say many years. It's annoyed me for a long time that his character changed so much from the bad boy he used to be, but I think this storyline may be an attempt to explain that.
