Yes, there is a factor of luck in this game, but with enough knowledge of how the game works you can control it. Knowing that Luigi has an opportunity to misfire his forward+b should mean that you put yourself in places where the misfire can't hit you. You know that Peach can pull stitch-faces and bob-ombs, and even though you can't control the turnips, you should be controlling the match to limit the odds of her being able to change the match's outcome with a single down+b.
It's harsh, but there you go.
It's harsh, but there you go.