(macOS)
According to Mullvad, “All of your DNS requests get rerouted to our own non-logging DNS resolver instead of to your ISP or a public DNS provider.” I use the Mullvad app on my Mac.
This makes me think that I don’t need to enable AdGuard’s AdGuard DNS on Mac, so I currently have DNS Protection disabled on the AdGuard App (and the radio set to System Default instead of AdGuard DNS, although it’s greyed-out since DNS Protection is disabled).