I don’t use the Magic Mouse because I think it’s uncomfortable, but I get notifications with my Apple keyboard and Logitech mouse, and there are native battery widgets in the control center that show for all of them (including non-Apple devices like my mouse and headphones). I find it very hard to believe that the Magic Mouse is unique and refuses to let people know its charge state.

Whether or not that notification pops up and disappears while I’m not using the computer is another story, but I’m pretty sure those notifications can be changed to stay on-screen until they’re dismissed.