Heat a skillet over medium-high heat. Add the beef and cook, breaking it up with a spoon, until browned, 6 to 8 minutes. Spoon off and discard drippings.
Stir in the cumin, 1 cup of the salsa, and ¾ teaspoon salt. Cook, stirring occasionally, until heated through, 2 to 3 minutes.
Fill the taco shells with the beef mixture and top with the avocado, sour cream, cilantro, and remaining ½ cup of salsa.