How important is a persons words(what we say)? We are not supposed to lie, ofcorses. However, can we say something in jest? Example- I was wearing sunglasses the other day, for the first time, since I have been in Chicago. About a half dozen people or so, called me "Joe cool." So, I finally said to the last of the dozen or so people, 'I am just going to legally change my name to 'Joe Cool.' Biblically, we are justified by our words. So, am I obligated to change my name? Is their no room for levity? The spirit did tell me to repent of all the promises that I have not kept. So, what say you? Words are very important, of course. The kind of speech we are to avoid is breaking promises, lies, cheating, dishonoring speech, using God's name in a purposeless way, and speech that glorifies sin.
But, as you say, what about joking? Well, we know that Jesus and Paul both used language as hyperbole-- exaggeration for effect-- and that was okay. Jesus was not actually encouraging us to cut off our hands or feet, although he used language that literally meant that. Rather he wanted us to know that sin is very serious and we should separate ourselves from anything that causes us to sin-- but not literally cut off body parts. Paul said of his opponents who taught the Gentiles to be circumcised, "I wish they would just cut it all off"-- i.e. be castrated. Did he mean that? Obviously not, it was humor and of a pretty sick kind really. Paul also said, "Among the Creteans, even one of their own said they are all foolish and liars." Of course, the joke was that if a Cretean said that all Createans were liars, then he was lying. It is an ancient joke that Paul was making reference to, but he did not mean to literally say that ALL Creteans were liars, even if he did make a racial slur.
So are jokes okay? As long as you are sure that everyone understood it not to be taken literally. The problem is that many jokes aren't understood-- especially if you are putting someone down in a joke. Many people don't think one is really joking, and, of course, many people when they make "jokes" like that aren't actually joking. It is most important that our jokes which concern promises and dishonor are absolutely known by everyone to be a joke. Then it's okay.
That's how come I try to make my jokes so over-the-top exaggeration that no one could possibly take it seriously. Also my jokes tend to be jokes about fact, rather than about promises or putting people down. So I would joke about "The sky is green" rather than joke "I'm going to give you a million dollars" or "Your face looks like it got in a car wreck." I do make jokes of the other two catagories, but, again, I joke so over the top no one takes it seriously.