“Just kidding” is the undo button people hit after they’ve already said what they meant.

The line arrives after the content has already landed, which is why it repairs so little. It also puts the other person in an awkward spot, because objecting now makes them the one who cannot take a joke. That is the function, whether or not anyone intends it consciously: the thing gets said and the responsibility gets waived.

Watch for yourself reaching for it. If you needed the escape hatch, the remark probably meant something. And when it is used on you, it is fine to say the joke landed badly and leave the silence there.