For user application it does not signal anything I guess, but I'm not sure if that's important for databases. Databases are usually crash-consistent, which is equivalent to a snapshot.
fsfreeze is unnecessary for device-mapper devices. The device-mapper (and
LVM) automatically freezes a filesystem on the device when a snapshot
creation is requested. For more details see the dmsetup(8) man page.
However, it does signal to the filesystem, quoting https://manned.org/fsfreeze.8 :