The relevant game tags are changeling-lost and changeling-dreaming. (Though perhaps they need to be changed for changeling-the-lost and changeling-the-dreaming, that's a tangent.)
I'd say it's borderline. I don't think there'd be any objection to a elf tag, so perhaps a changeling tag for the D&D race isn't uncalled for. On the other hand, elf is really a broad tag that applies to a concept that's not limited to D&D 4e, while (the D&D conception of) changelings aren't broadly relevant beyond the game they're in.
However, I do note that in practice the only questions that have elf on them are two D&D 4e questions, which might indicate that even elf is unnecessary granularity.
Judging by the current practice I'd say that race-specific tags are problematic because: a) they are useful in the scope of D&D 4e rules questions, because powers are built around things like class and race, but b) such tags are going to become D&D 4e specific as questions about other games have little reason to use race-specific tags like that.
Calling such tags problematic doesn't mean we should get rid of them, but it does mean they don't fit into our tag system very well.
