You can most likely leave the DNS server of mullvad as second priority so you have a backup if your AdGuard container is not reachable
That’s not really how DNS works. If you have two DNS servers configured, the OS will usually do either one of two things:
Balance the requests between them; or
Send the request to both servers and use the one that replied faster
If you mix Mullvad and AdGuardHome DNS servers, you’ll very likely end up with a mix of both being used. The DNS servers configured on your clients should either be all AdGuardHome or all Mullvad so that you don’t confuse yourself (“why are some ads blocked but not others??”) :)
If you want to continue using Mullvad’s DNS servers, that’s fine, but you’d set them as upstream servers in AdGuardHome, and then configure all devices to use the AdGuardHome DNS. Ideally use their DNS servers using DNS-over-HTTPS or DNS-over-TLS: mullvad.net/en/…/dns-over-https-and-dns-over-tls