Allied propaganda in WWI showed the Kaiser as basically the Demon, bloodthristy, cruel, slave owner, etc.
But in WWII, the propaganda mostly was about showing the Nazis as idiots, fools, mocking them and basically making fun of them
Why was that? why did the propaganda turned so..opposite way?
I'm not really sure I'd agree with your assessment of WWII posters being so different. From the hundreds I've studied, there are more of them I'd say take a serious tone than take a humorous one, as well as plenty of caricatured depictions of the Kaiser from WWI. What posters are you referring to that give you the impression WWII posters made a distinct change?