In my exploration of Mexican cuisine, I discovered that the tradition of putting sour cream on tacos is more common in the USA than in Mexico itself. In Mexico, they typically use a variety of fresh and often spicy ingredients such as salsa, cilantro, onion, and lime. While some might use crema, a Mexican version of sour cream, it's not as common or as thick as the American counterpart. So, while you'll see sour cream on tacos in many American-Mexican restaurants, it's not the traditional topping you'd find in a Mexican home. It seems the creamy addition is a delicious adaptation that has grown popular outside of Mexico.