South Park Pathology refers to an attitude characterized by a deliberate display of apathy or disdain for caring about something, stemming from the belief that showing enthusiasm or concern is uncool or cringe. This mindset is often presented as a way of being "above" the act of caring, and it was popularized by the animated television show South Park. The term is particularly associated with millennials and serves as a critique of "poser" culture: if it is considered uncool to care deeply about something, then, paradoxically, it becomes "cool" to appear disinterested or dismissive.
John is embarrassed to tell people he plays Yu-Gi-Oh! because of South Park Pathology, fearing they might think it's cringe to care about a card game.
John is considered "based" because he shares his interests authentically and passionately, even though others with South Park Pathology tell him he's cringe for caring too much.