Using a food processor or blender, puree mangos until nearly smooth. Transfer to a large bowl and add buttermilk, minced peppers, and ranch salad dressing mix. Mix on low speed with an electric mixer until well combined. Add mayonnaise and continue mixing at low speed until dressing is smooth, about 2 minutes. Refrigerate until dressing has thickened, 3 to 4 hours. Stir before using and store in the refrigerator.