Ms. Harman really doesn't get humour does she?
She wheels out an ancient rag-mag joke as an example of the sort of thing she used to 'slap down' - which got her accused of being a humourless feminist.
In that case, she would have been better recounting a sexist joke, which would have been unlikely to cause such a furore, given that anti-Semitism is probably just a tad higher in the Labour alarm scale than sexism, but it does depend on what day of the week it is, or something like that.
She is humourless, because part of having a genuine sense of humour is knowing where, when and how to exercise it, and on this occasion, she fell down badly on all three counts.
Suggesting in advance that your audience is going to be amused is either the stance of a professional comedian, with a tried and trusted set of jokes that are going to work.
Saying it as a politician on a live TV show demonstrates serious lack of judgement, and underlines the accuracy of the accusation - Ms Harman is indeed devoid of a sense of humour.
That rarely did Margaret Thatcher any harm, but at least she had the sense to keep 'humour' out of her public encounters.
You can get away with saying things like 'Every Prime Minister needs a Willie ... ' as MT did referring to Willie Whitelaw, because she was too humourless to see the amusement in what she said.
But trying to be double-bluff clever as Ms Harman did, shows not only an absence of humour, which is allowable in a politician, but an absence of judgement, which is not.