Just because you couldn't lie doesn't mean that you have to tell the truth in a nasty way that would piss the other person off.
But yes, people would get their feelings hurt though.
People would find out if others think they're overweight, or unattractive, or smell odd.
But it's better to find out about it and be motivated to change it (exercise, plastic surgery, showering regularly), then have everyone be thinking it but not tell them.
I think it would be worth it in the end.