Possibly the reason some don't find it amusing is because for the last I don't know how many years now, we appear to be living in a society where such behaviour is becoming in reality, quite commonplace.
There's bullying, harassing and intimidation of MPs (not to mention death-threats), and to ordinary people on the streets... threats on social media against high-profile individuals by those who disagree with them, etc...
This, actually, is not a new joke - I've heard a variation of it before.
We might have been able to laugh more easily decades ago, but there seems to be a nasty underbelly of violence at the moment.
Much as I dislike Boris Johnson... I didn't really find it funny either.