If complete and utter chaos was lightning, then he'd be the sort to stand on a hilltop in a thunderstorm wearing wet copper armour and shouting 'All gods are bastards!

The line is a hyperbolic character sketch: it imagines “chaos” as a lightning strike and then escalates the image to someone actively courting that danger—standing exposed on a hilltop in conductive armor—while hurling blasphemy at the heavens. The humor comes from the absurd specificity (wet copper armour) and the audacity of the shouted provocation, which together signal a personality defined by reckless defiance, contempt for authority (even divine authority), and a taste for self-destructive spectacle. In Pratchett’s mode, it also satirizes grand, romanticized rebellion by pushing it into farce: the would‑be iconoclast isn’t merely nonconformist; he’s practically trying to get smitten.