I like to believe otherwise, saying "that group of people are notorius" is just too convenient. I've seen many locals stand up from a picnic in a city park in Munich leaving their rubbish behind.
My experience in Berlin from approaching people littering is that they're mostly not German. And what's fascinating, they're actually proud of their littering.
Caveat: we're probably both suffering from https://en.wikipedia.org/wiki/Confirmation_bias .