It doesn't have to be illegal. Naoko rubber-stamps licenses for pretty much
anything and everything, so there is no reason why there can't be an official Sailor Moon VTuber. The offer would need to be made by some big-name company, of course, not by some random YouTube person.
On the other hand, out of all hundreds of VTubers out there, I don't remember seeing any originating from previously existing franchise, or assuming personality of some established character. Even though it sounds like a lot of easy money with minimal investments. Not sure why.