I'm using Umbrel for several things and I'm surprised it's not there yet. It would explode the number of Joplin Server users imho. Setting up Joplin Server on a raspberry pi would be a few clicks away.
There seem to be an unofficial app though see GitHub - dennysubke/dennys-umbrel-app-store: Community App Store for Umbrel. This store is not maintained by the Umbrel team.