Jokiness is the noun: the jokey quality of a person, a remark, or a whole piece of writing. You might talk about the jokiness of his speech, or the jokiness of a film that never sits still and is serious. It names the tone that runs through something, not one single joke. It is not a common word. People more often say humour, jokes, or “he is always joking.” Use jokiness when you want to point at the whole manner — that light, not-serious flavour. In a kind sentence it is just a description. In a sharp one, jokiness can sound a little unkind, as if the jokes got in the way of something that needed to be straight. There is no everyday need to learn to say it yourself; you mainly need to recognise it when a writer names that tone.