There is a different use case though. Keybase already has your Public Key so to prove you own a domain it is sufficient to put a signature up and available in

There is a different use case though. Keybase already has your Public Key so to prove you own a domain it is sufficient to put a signature up and available in DNS to validate this claim. An attacker does not have this validation signature and can thusly not arbitrarily sign sites as you. I don't see how keybase would help for a Certificate Validation, especially since Keybase makes no actual attempts at indentifying you (that is a problem of your social circle on keybase)