I think a lot of it would depend on where you worked as a nurse. If I was working in a school, or admin at the hospital, I'd dress up to a degree. However, in ICU, the most I'd do would be a fake "tail" hanging out from my scrubs
or some cat ears that I could rip off in a crisis. NO face paint....if I woke up in ICU and saw Batman looking at me, I'd scream down the house, thinking I'd lost my mind.
I'm kinda conflicted about the Christmas thing. I had a friend who's son died the 21st of December. Well, of course, the funeral home had Christmas trees up. I thought, "oh, that's creepy and insensitve, let's remind her that her son's dead for Christmas..." but then I though, Nan already knows her son's dead and it's Christmas. Later, she said she liked that there were trees and wreaths, because her son loved Christmas. At my old hospital, we always had local churches and schools
that would come thru, caroling and handing out candy canes. The patients loved it. Had someone coded, we'd have got them off the unit, quick.
If you decorate, you're going to offend some. If you don't decorate, you're going to offend others. Just do what's safe and your facility lets you do.