Like with Peach, the bio will explain her title.
The 30th Century, an era of peace and tranquility, of hope and Serenity. Yes, the capital “S” is on purpose. This is the era of Neo Queen Serenity, Usagi Tsukino I, the original Sailor Moon, the ruler of Earth from Crystal Tokyo. However, that peace was shattered once the Cybermen infiltrated Crystal Tokyo and kidnapped Neo Queen Serenity, altering her into Cyber-Leader Gi.
Thanks to Moon-kyung, Lacey, their team, the Sailor Senshi of the solar system, the new Sailor Moon, Usagi Tsukino II, and the Asteroid Senshi, Neo Queen Serenity was freed, splitting her from Cyber-Leader Gi and making her into Kamen Rider Crescent. Because Megumi is the Queen of the F.N.S, Usagi I agreed to being Dame Usagi.