As this is a fan forum for Sailor Moon, and that means all versions, we do not have a problem with you choosing a username that is directly related to the characters. However, please do refrain from using your username to "assert" yourself over other members. (Ex. If you choose Sailor Moon, do not claim to BE the only Sailor Moon, just as we Admins do not claim to be our character counterparts.)
This is just a fun place for discussion. We will not allow anyone to be bullied here.
If you choose a username based off the show, please just remember to show respect to that character by not doing something against their personality. That is why the Admins have chosen the tags we have - because we use the character's personality to keep a clear head.
Thanks you,
~Sailor Cosmos