Ubuntu – Broken snap core module

You can't remove the ubuntu-core snap using the snap command, no. If you really want to remove it, remove the snapd package itself and then remove the /var/lib/snapd directory (if it says it's still in use, unmount the snap and try again).

Each individual snap is a read-only squashfs filesystem image. In order to access files within these images, snapd mounts the images, one for each installed snap version, inside /snap.

This list of mounted loop devices thus effectively includes the snaps you have installed, and is part of the normal operation of snapd. You should not attempt remove them. If you really don't want them there at all, then you can remove the corresponding snaps.

Various utilities are designed to show you all mounted filesystems, including loopback ones, which is why you see them. snapd is the first common daemon I've seen to maintain a large and variable set of loopback mounts in this way, which admittedly does hamper the UI of these utilities. Perhaps eventually we'll see these utilities hide loopback mounts by default.