"Have a nice day" has multiple meanings. It can be a harmless but trite valediction, it can be used sarcastically, and it can be used to rub in bad news. My lawyer rang me and said, "Expect to be served a subpoena in the next few days. Have a nice day." I replied, "You are fired, but I'm already missing you." Sarcasm is not dishonest towards those who can identify it. Using sarcasm on people who you know won't understand it is not very kind. The same goes for humor, which is easily misunderstood. Different courses for different horses.