I think the more recent usage is almost a backlash against healthy eating drives by the govt, Jamie Oliver, the purveyors of organic food, etc. Anything I’ve seen that’s referred to as “dirty” appears to translate to any foodstuff overlaid with excessive sugar, salt and fat (which is always going to taste great ). Perhaps better terms might be “Guilty”, “Immoral”, “Shameful”, etc. - in fact anything but a term indicating insanitary practices. Would anyone buy a “filthy” or “diseased” burger?