I do suspect this might be a better question for /r/movies; as far as I'm aware none of us here are film critics.

That being said and with the caveat that I'm not a film expert, I think I would challenge the assertion. For one thing, "serious" isn't a genre. But, which "serious" movies in which decade do you have in mind that don't have moments of humor?

Looking at the Academy Award winners for Best Picture since 1940, for instance, I would describe every single one of them that I've seen (most but not all) as being serious but having humorous moments.

To pick just one such winner from each decade, movies like Casablanca, Bridge on the River Kwai, A Man For All Seasons, The Godfather, Amadeus, and Braveheart, I would describe all of these as "serious" movies, and they all have moments of humor throughout.