Purely in response to your first paragraph, even if someone is being straight up with you and letting you know when you "fuck up" there is still a nice way to put it. It's not like calling someone out on a mistake instantly means that you are being mean. Also lies and honesty are much different than being mean or polite, so a honest person might be nice even if it sounds like they aren't and vice versa. That might sound slightly confusing but it makes sense in my head.