Inside-jokes are typically lame and generally unfunny to everyone but the few who understand them; culture inside-jokes are likewise invariably stupid yet accepted by most in a given culture as excellent.
Cultural inside-jokes need not be about comedy! Anything and anyone can be a cultural inside-joke. The only requirement is that the appreciation for the thing or person extends far beyond reasonableness and clearly reflects the sentimentality and bias of a given culture.
If you want to see an excellent example of a cultural inside-joke, just head down to a Modern Art Museum!