> That given he is comedian, his reporting is less credible or audience should receive his stories less seriously or what?

As he is primarily known as a comedian, there is no way to confuse him as a journalist doing "news reporting". This calibrates the critical thinking dial in my brain.

> Given that he throws a joke from time to time, does it mean that it's less reporting and more comedy?

Have you watched the show? The joke to content ratio is very high.

you are confusing jokes with fiction. It is possible to tell true story with additional jokes there and there without fiction. And I think here is confusion - for me Last Week Tonight seems like serious show which tries to do a lot of research to present some facts to audience in entertaining way.