I've had a fraught relationship with a parent, and I can understand how the internal justification behind this decision might go. Some shade of “maybe it'll go differently this time” even when already sure that it wont. And Kurt is a superhero who has dedicated his life to helping others improve theirs, so the feeling that it costs nothing to be compassionate, and that it's the right thing to do besides, is likely to be a potent one in his position.
My issue is consequently not with the abstraction of him doing something nice for Raven, while knowing she probably doesn't deserve the consideration, but with the actual circumstance of the vote. Namely, it makes him look deeply foolish.
Kurt is quite aware of who Irene Adler is. He knows that whatever is about to happen, it's more than likely going to be exactly what she wants. He should be aware that his vote is therefore one with the potential to be decisive, both to the motion and to the future of his society, and not something to be thrown away just because it suits him in the moment.
I've seen some fans float the idea that maybe Kurt just wanted Destiny there, for some reason. Well, maybe, but there's not really any sign of it in this story. Hickman has set things up for other authors before in his books, so possibly he just did this for Si Spurrier without being fully informed as to why, but I think it reads more like Kurt just made a big mistake.
As to how significant that is, while Kurt has recurred fairly regularly in Hickman's run, he is not one of the main characters within it. So to that extent I don't think an isolated character glitch here is that big of a deal—compared to say, in Way of X, where he's the co-lead alongside Legion—but it's such a monumentally consequential error, connected to both Destiny's ascension and Moira's exile, in the highest profile book of the year, that it's the kind of thing that could easily define his character in the near term if they're not careful about it.