I would say you don’t need to include it in the backup as your photos are stored in the cloud and will just repopulate the app if you had to re-install. However, if you have any Albums stored locally or any photos or edits that have not synced, then you may lose these.
Therefore, I would suggest trying to reduce the size taken up by the app by trying to Clear Cache. You can do this by tapping on the Lr logo (top left) and going into Local Storage and by tapping on the three dots to the right of all Photos. Maybe try both. It’s probably best to ensure there Is no outstanding syncing.
This should just limit the photos on the device to either thumbnails of Smart Previews (unless you have any Albums Stored Locally, which you might want to check).