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
I honestly think CLI control is essential in any app. Because this is the most rudimentary and accessible way for other apps and scripts to interact with the apps features and control over the system.