What do you mean by important? if you mean narratively speaking, after One Piece ends we look back at everything.
No, Kaido and Big Mom have a very clear and minor purpose, they are just in the story to serve as a roadblock, to be a luffy fight for the plot rocks.
fighting them is important, but it's just one more step in the overachieving narrative all the way to taking down the WG, they still are a pivotal part of the manga, where things change a lot after their defeat.
but comparing it to the void century, Imu, and so on, they aren't relevant in the grand scheme.