I have asked myself this question before too!
I think if it is clear to the other person that it is a joke, then it is a joke, not a lie, right?
For example… hm, let me think… ok, let’s say somebody who is not a party person at all says, as a joke: "I am going to [some big famous party] tonight. Kind of making fun of him/herself, because it is clear to everybody that it can’t be true.
However, I am just asking myself now - if you make a joke and for a while the other person WOULD believe it - but it’s just to laugh about it later - let’s say a daughter calls her Mom after she got her hair cut, and says, as a joke: “I died my hair green!” And the Mom is shocked, and says: “Really?”, not knowing if it was a joke or not. And the daughter says, laughing, “no, I was just kidding!”
And how about April Fool’s Day jokes?
Kathrin