The one-line command I recommend for install Mullvad's RPM repo is as follows:
curl --tlsv1.3 -fsS https://repository.mullvad.net/rpm/stable/mullvad.repo | pkexec tee -a "/etc/yum.repos.d/mullvad.repo"

My explanation: This curl command enforces strong TLS encryption and pipes the fetched repo file to the tee (append) command, which requests to run with root permissions and appends the file to the specified path. pkexec is useful instead of plain sudo because if the current user isn't in wheel/sudo groul it requests the local admin account to authenticate.