What do you mean by beeing offline? Offline as no internet, but access to your Nextcloud server on you LAN, or no connection at all, and your Nextcloud server is running on your client computer?
Not sure, probably not, but there are desktop apps that will work offline, and It’s also possible to host your own instance using Docker and then point the draw.io app in Nextcloud to that: GitHub - jgraph/docker-drawio: Dockerized draw.io based on whichever is the most secure image at the time.