If you launch mullvad from the app icon, try you add to PrefersNonDefaultGPU=true (or false) in mullvad-vpn.desktop (cp /usr/share/applications/mullvad-vpn.desktop ~/.local/share/applications
If it is autostarted add it in .config/autostart/mullvad-vpn.desktop
Or add –disable-gpu to the in Exec : Exec=“/opt/Mullvad VPN/mullvad-vpn” --disable-gpu %U