Through 3 different adaptations of the series, Princess Serenity's death has been portrayed in 3 different ways. In the original manga, the young princess commits suicide when Prince Endymion died protecting her during Moon Kingdom's downfall. In the anime, both perished in the attack, while in the live action series, it was the Princess herself that destroyed Moon Kingdom out of grief, as Endymion was killed during the war to keep the two apart.
It doesn't really matter how you're heard her story, I'm pretty sure a lot of people still know Sailor Moon, even after all these years. However, I'm surprised someone is still cosplaying her in Comiket! And such a beutiful dress too. Nice cosplaying!