The way I see it is that Mr. Park did not say that Mr. Kim smelled bad at all. When Mrs. Park asked what kind of smell was it, if it was like an old rag being boiled or old man's smell, Mr. Park said it's not those specifically, it's the smell of people in subways. Which to me isn't saying about Mr. Kim personally smelling bad, but that his smell belongs to a certain class of people.
@Athena is right that the young Park son was actually the first one who noticed the smell, but the son also never said anything or implied that it was a bad smell. I find it's interesting that some in the audience may think that when referencing and/or talking about a person's smell, that it automatically means a person smells bad.
I agree that while Mrs. Park only recognized the smell after her husband mentioned it to her, it also probably didn't help that at that particular time in the car, Mr. Kim might have actually smelled bad, considering he weathered a storm the prior night, slept in a gym and put on clothes that came from a pile of donations. I would wager he probably didn't get a good shower in the gym as well, just washed enough to look clean.