Well, it depends.
You probably think 90% is pretty good because you're comparing yourself to the people around you. In actual fact, national (USA) seatbelt compliance is around 91%, with the great majority of people wearing a seatbelt ~100% of the time, and the skew being made up of a minority of people who are wearing a seatbelt ~0% of the time.
Now, I can't speak for what the other person is thinking... but objectively, most people are doing better than 90%. So when you say 90% is pretty good, it seems to me that they're doing you a favor by telling you that it's not. If you go around thinking that 90% is pretty good, you might not change, right?
It's not necessarily a moral judgement. It's just a fact. By telling you that 90% is not pretty good, they're helping you, as well as everyone you share a car with.