Seems to me to have always been a meme, even before the internet. I suspect it's some amount of shit-talking going to an extreme, propaganda that has long just turned into an incomprehensible slush, or it's adopted from other countries' hate of france being treated as self-explanatory and just becoming a given.

Very rarely is the hate for france real, it's pretty much just a joke anytime it's been used while I've been in earshot.

The joke template is: Anyone: *says something disparaging about the french that is either very vague or about them being snobbish or quick to surrender*

Everyone else: *agrees without question or elaboration.*

I'm sure some portion of the US genuinely does view the French in a bad light, but I've yet to meet them.