don’t know why you’d want to? you may trust your dns server but without dns over https the dns requests themselves are sent plaintext and are vulnerable to man-in-the-middle attack. with dns over https the dns requests are encrypted and that encryption would have to be broken for a MITM attacker to see your requests. more security is better and dns over https costs virtually nothing to use in terms of cpu resources.

edit: oh do you mean whole system mullvad VPN? if so, then yeah dns over https doesn’t really help much but it’s also still a case of why bother turning it off when there’s no benefit to it.