I would like to put a raised garden about 2x8 feet 6 inches deep but the area is covered in thick moss. If I cover it with a pourous plastic sheeting under the garden dirt will the moss die? Is a shady area good for tomatoes and vegetables? Currently it gets some morning and afternoon sun
Moss is usually a sign that there is too much shade for anything else to grow. Think of old trees, moss is usually found on the North side of the stems where the sun don't shine.
Vegetables and certainly tomatoes need a lot of direct sunlight. I think you'll have to reconsider the location for your vegetable garden.