Shantae's magic tiara "is supposed to be an endgame bonus for getting all the gallery keys, but considering the keys aren't any harder to find than the other items necessary for the good end, it's very likely players will have this before they set foot in the final world"
I doubt that It could be used in the canon or this fight (or any fight really because it literally makes her invincible whit bubble and that would be a big, fat STOMP).
It's like giving some RPG game character 'immortality in our abilities page because you can obtain this "'one secret item that you're supposed to get after beating main story" which gives you infinite health.