Preheat oven to 400°F and position a rack in the center.
Cut the stem off each of the 26 mini bell peppers and cut each pepper in half lengthwise. Remove seeds and white membranes. Arrange all pepper halves, cut side up, in a single layer on a baking sheet.
Heat a medium skillet over medium-high heat. Add ½ pound lean ground beef and cook, breaking the meat into small pieces, until no longer pink throughout, about 5–7 minutes. If there is noticeable excess fat, carefully drain it from the pan.
Stir 6 tablespoons salsa into the cooked beef until evenly combined.
Spoon the beef-and-salsa mixture into the pepper halves, dividing the mixture evenly among the peppers.
Evenly sprinkle 1 cup shredded reduced-fat Mexican cheese blend (packed, 4 ounces) over the filled peppers.
Bake in the preheated oven until the cheese is melted, about 5–7 minutes.
Remove the baking sheet from the oven and top the peppers with ½ cup diced tomato, ¼ cup diced green onion, 1 large avocado diced, and chopped cilantro to taste.
Serve the mini bell pepper nachos warm.