The reason why Fuwawa loves yobai is because she yobais Mococo regularly. We already know she sometimes night crawls to her bunk and hugs her (
https://youtu.be/Y472rC3A9eo?t=9408), and that Mococo is always the little spoon (
https://twitter.com/fuwamoco_en/status/1699413360960479412). The jump from that to yobai is not that big. There's no other reason for why Fuwawa would be so obsessed with a singular word other than the fact that she engages in it regularly. It's yobai in plain sight.
(P.S. Mococo's bunk is bigger (
https://youtu.be/AL3FAfr8FGc?t=3662) because it's better for yobai parties)
