The Magic Mouse is an example of something that looks like poor design, but honestly isn't.
1. You can't use it while charging, which is by design. That should be obvious.
2. It's supposed to be a wireless mouse, and leaving it plugged in all the time is what most people would do. So ergonomically, forcing people to unplug it to use it is the intended use case.
3. Leaving the mouse plugged in all the time would also have implications for the battery; constant charging would be bad for it. Letting it drain while plugged in would be confusing.
Should Apple have made a charging dock or something similar? Probably. But making it impossible to use while charging is exactly what was intended, and it was executed quite well. And I've never met anyone who has one that cared one lick about it.