Tough call. You certainly do have the right to tell the person its not their business. They might not have noticed what they were doing, but if I was to the point where strangers were going out of their way to tell me that my clothes don't match, I smell bad, am too big to be eating McDonald's, etc, I certainly would give some thought to what they said.
Certainly if there is one person who is actually willing to step up and say something, it's highly likely that everyone else around is thinking the same thing, but are too polite to say anything.