After your media library has been synced to Sirv, it is safe to delete the WordPress generated thumbnails. This will save a lot of disk space on your WordPress server. Go to the Image deletion section of the Synchronization tab in your Sirv plugin settings. You can easily regenerate the WordPress thumbnails in future, if needed.
Your original images should be kept in your WordPress media library - it is not safe to delete those. The Sirv plugin regards those images as the masters. If your Sirv database cache was ever cleared in the future or it somehow became corrupt, the plugin would no longer know which original image should be mapped to which Sirv image.