I can't imagine too many situations where being mean would be the most effective course of action.
I think it's important to be truthful, and oftentimes the truth can hurt people's feelings and make them upset... but it's usually short term pain that helps them in the long run.
I guess if someone is harassing you or trying to make your life impossible, then being mean might be warranted, but again, I can think of far better strategies when it comes to dealing with difficult people like that.
All in all, it really comes down to what you think is OK or not, but I imagine that kindness is usually the best way forward, and if it doesn't exactly work out as you hoped, at least you know that you were still a good and decent person that didn't have to resort to upsetting people for no reason. There's always a better way.
Great question dude! Hope you're well!
In most situations, I would say no, it is not alright, but there can be minor deviations from this.
When people are bothering me in some capacity, I'm not always direct in every aspect of dealing with them, because of my friendly nature, but I try to avoid their attempts at hanging out, because I am busy. Some people are not adept at reading subtleties and body language, and will continue to press you, even when you politely tell them no.
I am sometimes a zero to one hundred kind of guy, in the sense that sometimes I kind of snap on these people to bring them into the reality of what no means. Sometimes I'm not met with the nicest reception, but generally, they are apologetic and try to figure out other ways to get their way,it's usually a person that has already been highly irritating me for a while before I've come to this point, and I've already prepared in my mind ever possible recourse I can think of.
I don't think it's necessarily right to be mean, but you should always advocate for yourself, in a friendly manner, and if people still push your boundary, you may find yourself being more vocal with your answer. These types are extremely difficult to get through to, and I would try to distance yourself from them.