Preheat oven to 375 degrees. Lightly brush both sides of tortillas with melted butter and sprinkle both sides with granulated sugar. Lay tortillas over 6 jumbo muffin cups (tortillas will overlap). Bake until tortillas are soft and pliable, 3 minutes. Remove from oven and, with the bottom of a glass, gently press each tortilla into a muffin cup so it folds up and forms a cup shape. Bake until tortillas are crisp and golden at edges, about 12 minutes. Immediately remove tortillas from cups and let cool completely on a wire rack, 15 minutes.
Meanwhile, in a small bowl, whisk together yogurt and 2 tablespoons brown sugar. In another small bowl, toss together fruit, 1 tablespoon brown sugar, and lemon juice. To serve, divide yogurt mixture among cooled tortilla cups and top with fruit.
Use a juice glass or ladle to form the tortilla cups. You can make them a day ahead: Let cool completely and store in an airtight container at room temperature.