Personally, I think it is nuts. Green Bay is home to lots and lots of Christian churches. I think a good place for nativity scenes is on the front lawns of those churches. The reason there was an original nativity scene, after all, was not that Mary and Joseph went to Bethlehem to worship. They were ordered to Bethlehem by an unpopular government and they weren't too welcome when they arrived.
Putting a nativity scene at the very center of a city government may not be the best way to recall the original reason for the season.