Why does the tag "whocares" exist on Hubski? Is it so users can choose to ignore/mute those posts, in the hopes that some content police will tag certain posts before they have to see them?
What are the conditions a post needs to have in order to get this tag? Are the rules published somewhere?
Who decides if the post meets those conditions?
There are no content police and there are no rules. The community tag exists so that people from the community can add either categorization or editorial to a post. In the case of #whocares, a tag that fwiw I've never used, it seems to be both. It's worth mentioning that if someone thought it an inappropriate community tag they could change it and if more people agreed with the changed tag than the #whocares tag the more popular suggestion will prevail. If you "ignore" #whocares (EDIT: or follow it), then yes somebody is curating on your behalf. But I doubt many people are using it this way. My guess is that someone is trying to editorialize, to let you know they think whatever is tagged that is trite.
If you disagree with their assessment, I wouldn't let it bother you. I mean, who cares?