She is known by such an endearing moniker simply because of her evil and manipulative ways. She is known for causing misery and violence to all who cross her path. In other words, she brings mischief into her opponents lives. This beauty encapsulates the entire concept of mischief with her emerald green eyes and flawless red hair, two traits associated with both mischievous persons and witches. Both of which she proudly admits to being.
The personification of evil, Sarah Twilight completely and undoubtedly has earned the title: Mistress of Mischief.
"Sarah Twilight is the Mistress of Mischief...she defines evil!"