Technically no, although it is somewhat rare to have an all out total wash out that MK fireworks are cancelled all together. They'll try to make every effort to run it eventually. But obviously anything can happen.
Don't go in expecting anything from this but if the weather REALLY hands your lemonades, it's not a bad idea to discuss it with guest services.
Someone recently posted about it raining on their night and they said they let people hang out in the dessert area (where it's covered) until the rain passed and the show eventually stated. That's a lot better circumstances than everyone else in the rain!
