Mullvad has Deb and RPM repositories now!
Install Mullvad app on Linux | Mullvad VPN
https://mullvad.net/en/help/install-mullvad-app-linux?q=linux
How to install and uninstall the Mullvad VPN app on Linux.
You can easily add them by following the instructions on their site.
On immutable fedora it can be done via
curl -o - https://repository.mullvad.net/rpm/stable/mullvad.repo | sudo tee /etc/yum.repos.d/mullvad.repo
rpm-ostree uninstall mullvad-vpn --install mullvad-vpn
# after reboot, if not working
sudo systemctl start mullvad-daemon