Running devicemapper in loopback mode is discouraged for production. It has known performance problems and a different storage driver should be used. See devicemapper performance considerations and selecting a storage driver to understand the available storage drivers and limitations. When possible, overlay2 is the preferred storage driver for all currently supported Linux distributions.
To bypass the warning add a bypass-storagedriver-warnings flag when running the replicated or operator install scripts. For example
curl -sSL https://get.replicated.com/docker > install
cat install | sudo bash -s bypass-storagedriver-warnings