No, Jellyseerr is a selfhosted webapp where you request movies and shows, which will be searched and downloaded by your configured Sonarr/Radarr, which will populate your Jellyfin.

You can see it as a user-friendly Sonarr+Radarr front-end, that you can sync with Jellyfin users, so they can go and request content directly without you manually adding things in Radarr/Sonarr.