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.
because it’s also owned by nazis. if ‘it’s just social media’ then go back to xitter. if you don’t want to because nazis, well…