Treat something as a (big/complete) joke
Despite how serious the outbreak is in some parts of the world, people across
the world (where nine cases have been confirmed) have been treating
it as a big joke, and I'm sick of it.
A Perth business owner says he's been forced to shut up shop after he was
bashed by a gang of teenagers.(…) "I don't think the juveniles are scared,
I think they're treating
it as a complete joke, " he said.
He added: “It would be better if Senator McCain sought to find solutions to
this challenge facing America rather than treating
it as a joke. "
Poke fun at, thumb your/his/her/their nose at, make a game of
probably sound a bit formal in conversational style, you're more likely to find
them in written style.