Preheat oven to 415°F (212°C).
Heat a large skillet over medium-high heat. Add the ground beef and the taco seasoning, breaking the meat up with a spatula.
Cook the beef, stirring occasionally, until no pink remains and it is browned.
Stir in the chopped olives, diced green chilies, and drained salsa. Season with salt and pepper to taste, then remove the pan from heat and let the mixture cool to room temperature.
Once cooled, stir 3 cups of the Mexican cheese into the beef mixture until combined.
Spoon about two heaping tablespoons of the beef-and-cheese mixture into each pasta shell.
Arrange the filled shells in a 9x13-inch baking pan in a single layer.
Top the filled shells with the fresh mozzarella and the remaining 1 cup of Mexican cheese.
Bake for 15–20 minutes, or until the cheese is melted and bubbly.
Remove from the oven and transfer the pan to a wire rack. Let cool for 5–7 minutes.
Top with cilantro, diced tomatoes, diced avocado, and sliced serrano peppers or jalapeños as desired, then serve immediately.