Although there is no mention of the wish list thumbs icons in Airbnb Help (that I can find), @Andy327, I do think it has to do with the sharing aspect of wish lists. I took a look at "How do I manage my list of saved homes?", and the idea is to share the saved list with the friends/family who will be travelling with you, and have them comment, add and delete. So the thumbs may be a way to vote among the group. I don't remember seeing those thumbs before today, so thanks for mentioning them. Interesting.
I hope getting thumbs down-ed in someone's wish list by the picky sister-in-law does not affect a host's standing. Yikes.