I don’t remember if there was a different canon statement on the relationship between the demons and the vampires, but I remember that the vampires were a weaker race that served the demon king and fought on the demon side of the holy war. Therefore I assume that most demons look down on vampires and this would have caused trouble for Gelda becoming demon queen, because the general demon population would look down on her and refuse to accept her. Except, that the alternative is much worse in the eyes of the demon population, because the alternative is Meliodas who married a goddess. Gelda looks much better as a candidate for queen of the demons simply because most demons are comparing her to Elizabeth. Gelda simply takes the win as the better outcome and Zeldris is annoyed that his lover isn’t respected for her own merit, but is trying to be pragmatic since, again, this is better than the alternative.
I imagine that once demons get to know her and see how well she does in her role they will respect her more for who she is, but there will always be other demons who attempt to make derisive comments about her only to get shut up by an elbow to the side and a harsh whisper of ‘would you have rather had that goddess’