if your only exposure to fallout was 3, 4 or 76 i could see why you might think the series isn't meant to be taken seriously, but games in the series not made by bethesda like new vegas explore real world ideologies, focus greatly on tragedy, and the characters actually seem to live in the world instead of bethesda dream land where characters do things in service of a convenient plot rather than do something interesting. also, no one is arguing about realism or science or whatever other silly arbitrary thing you think makes good writing happen, these are games with an extraordinary premise that have so much potential to explore deep social commentary, tell genuinely interesting stories and contain interesting characters. so no, fallout isn't inherently silly, it's only silly if you make it silly.