Yes.

It's the reason why Jolla managed to fast prototype and test a Sailfish OS Smartwatch.
They leveraged the hard work of porting mer to that specific hardware from AsteroidOS, and "only" had to implement the sailfish-specific UI layer they wanted to explore and play around with.

But I think that's basically everything that is more or less alive currently.

Nemo Mobile is a small scale community effort.

KDE Plasma Active wanted also to use Mer as a base for their Vilvadi tablet, but as the whole project went bust...

Speaking of KDE Plasma Active, Purism has decided NOT to use Mer for their Librem5 phone.
Among other reasons : they wanted to share a common core with their laptop OS which uses a Debian core.
And also, as they want to design and built a smartphone from the ground up (instead of relying on manufacturer-provided ready-to-use PCB designs), and build it with opensource in mind, they definitely not need android drivers adatpation like libhybris and thus don't need Mer that much.