I do see it as rude, though, not just a pet peeve. It is making an interesting assumption that the teller knows better than the person how she feels and that the teller should tell her what to do. It's not a precautionary warning, it's not a greeting, it's a command (even if given in a jovial tone) and it says "It doesn't matter what you're feeling today, I think you should smile, so do it!"