No, the Multi World interpretation of Quantum Mechanics is not fictional, but it is not real.
The fictional thing is a thing, which is announced by author to be non-existent. For example, if book is called "Fairytale", then the persons in the book are fictional.
If a monster is not announced as fiction, then it is not fictional.
But FSM is fictional, because:
1. It is absurd: spaghety can not fly, and pasta is dead.
Do you agree then, that FSM does not exist? Have I finally disproven the pastafarians?