No, Mysterium isn't, but Mullvad just partnered with Obscura to create an onion routing like service.
I asked Obscura if it worked with Tailscale here:
https://x.com/mdhardcastle/status/1949442089902547039
https://obscura.net/
https://mullvad.net/en/blog/mullvad-partnered-with-obscura-vpn