Not a single French unit broke and ran in WWII. The same cannot be said about American units.
The reason SmokinLoon and others respond is because the accusation of cowardness is brought up, as "a joke" of course, everytime the French are significantly part of a discussion. It isn't funny anymore because we've all heard it hundreds or thousands of times.
So no, it isn't about a sense of humor, it is about hearing the same misinformed quips, defensively passed off as a joke when called on it, over and over and over and over and over and over....
I don't believe people when they say it was a joke because most people I have talked about this with do believe that the joke is based on reality, that the French did lose because of cowardness, not because they were out manuevered and out thought strategically and tactically.