I can't see my Rocket League Sideswipe items in Fortnite

Your inventory in Sideswipe is not shared with your Fortnite account; each game maintains a separate inventory. There are some items that are shared between Rocket League and Fortnite, but Rocket League Sideswipe items do not sync with them. 

To see which items sync between Rocket League and Fortnite, see: How to check if a Rocket League item can be used in Fortnite.