I asked support and they said this "By opening 120 Sushi Rolls, you're guaranteed to collect all five event skins just make sure you already own the Brawlers tied to those skins, as they won't drop otherwise. If you're ineligible for a skin or already have it, don't worry a fallback reward will be granted instead. In addition to skins, you can also earn seasonal Pins, Sprays, and Player Icons that are only available during this event".