To prepare the salad, combine corn, bell pepper, butter lettuce, and mango. To make the dressing, combine mustard, balsamic vinegar, olive oil, lemon, salt, and pepper in a jar and shake vigorously. Alternatively, you can simply mix it in a bowl with a whisk, or prepare it in a blender.
Pour dressing over salad, add mint and basil, and stir to combine. Salad and dressing can be made ahead and stored separately for later serving.