Strictly speaking, it's anytime there's a difference between one genome and the consensus human genome. But since you're presumably asking whether we should co

Strictly speaking, it's anytime there's a difference between one genome and the consensus human genome. But since you're presumably asking whether we should consider a mutation to be good or not, that's really the same as asking whether anything is good. Is blindness good, for example? I'd say not, but others may choose to blind themselves if it makes them happy. But I doubt most people would welcome a genetic mutation that blinded them.