It's not funny in this situation because it's just a wrong use of the phrase. As a Gen Xer myself I have no problem with it being applied to people of my age or even younger, if it is used correctly. If someone is trivialising the genuine concerns of millennials or Gen Z, then it's fine to say "OK Boomer". Baby boomers are now famous for being blasé about messing up the world for future generations, sucking up all the money because of macro-economic trends, rather than any efforts they put in and ruining the environment. If someone calls a younger person a snowflake because they are complaining that they belong to the first generation to be worse off than their parents since forever, then that young person is within their rights to respond "OK Boomer". If a forty year old tells a thirty year old to stop whinging about paying 70% of their income out as rent, then "OK Boomer" is a fair response. Here it has been used as a synonym for "OK Granddad", insinuating that a person is older than their years because of a tendency to complain excessively, which is just wrong.