Preheat the oven to 350ºF. In a 12-inch non-stick skillet over medium heat, cook the ground beef until no pink remains, breaking it apart with a meat masher tool. Drain off any excess fat. Incorporate the taco seasoning and the can of undrained diced tomatoes with green chiles into the beef. Stir well and simmer for 5 minutes before removing from heat. In a mixing bowl, blend the shredded cheddar cheese and ranch dressing with the warm taco mixture until thoroughly combined. Distribute this filling evenly into the frozen phyllo tart shells. At this stage, the filled shells may be frozen for future use. Arrange the filled tartlets on a rimmed baking sheet and bake for 8 to 10 minutes, or until the cheese is fully melted and bubbly. If baking directly from frozen, extend the baking time by an additional 2 to 3 minutes.