I'm not familiar with Wikidata, but I don't think this would be ok.
When you go to create an article on Wikidata, you're met with this:
There's nothing there about contributing content that is licensed by others. It must be your own original content. So, no CC-BY content, no GFDL content, no all rights reserved content, nothing - unless it's written and owned by you, or already in the public domain.
I suspect the exact problem they're aiming to avoid is the one you stated: they want third-parties to be able to use the Wikidata content without fear of getting sued (having said that, there's still a risk if the person who originally contributed the data lied about where it came from).
Even though CC-BY isn't share-alike, you still can't release CC-BY content under CC0, because by doing so you would be nullifying any requirements for attribution to the original author - something you don't have the right to do. The original author still retains all copyright on their creation, and absense of a share-alike clause doesn't allow you to change the license on their work - it just frees you to not have to release your changes in a similar manner.
It's also worth noting that CC0 may not hold under some jurisdictions anyway due in part to the inability to revoke moral rights. As mentioned by Zizouz212 in the comments, this includes Canada and Germany, among others.