> Mullvad Browser uses them [ the DoH servers ] by default when you're not on Mullvad VPN, preventing your ISP from seeing the domains you visit.
This is a half-truth until Encrypted Client Hello (ECH) is in-use for most of your traffic. Sure, you won't have clear-text DNS floating out there in the series of tubes. However, without ECH the hostname you're connecting to is exposed in the SNI field during TLS handshakes.
I imagine most sites we are browsing will eventually be able to leverage ECH, so this should become less of an issue over time, but in the event you frequent any self-hosted operations (e.g. no Cloudflare or similar), encrypted client hello could still reveal identifying information since it requires still exposing an "outer SNI" that a valid certificate needs to be presented for. You only get to easily hide in with the masses when you use a big infrastructure provider as a consequence, since the common outer SNI of cloudflare-ech.com doesn't really reveal anything different than an IP <-> ASN database would. But if the outer SNI is "dunder-cat-enterprises.com", then all I've protected you from is your ISP snooping on the subdomain or pinpointing exactly which one of my domains you might be talking to.