Heat a large skillet over medium-high heat. Cook and stir beef in the hot skillet until browned and crumbly, 5 to 7 minutes; drain and discard grease. Mix taco seasoning into ground beef.
Combine water, Mexican rice mix, and butter in a saucepan; bring to a boil for 1 minute. Cover saucepan, reduce heat to low, and cook until water is absorbed, about 25 minutes. Mix ground beef into rice mixture.
Bring a pot of water to a boil; cook green bell peppers until slightly softened, about 5 minutes. Drain. Fill each bell pepper with beef-rice mixture. Arrange bell peppers in a baking dish. Pour ranch dressing over bell peppers and top each with Mexican cheese blend.
Bake in the preheated oven until peppers are tender and cheese is melted, 45 to 50 minutes.