The times that I have stuck wafer paper decorations on with buttercream, it absorbs moisture from it and wilts and loses its shape. For example, I did a cake once that had a written note on the bottom that I attached with buttercream. After probably an hour or two, the note had bent over so you couldn’t see the writing. One thing that I haven’t tried, but may be useful: try coating the cake in a (thin) layer of piping gel. This way it may wilt some, but hopefully not as much as it does with buttercream. I wouldn’t do it on the actual cake without trying it first though.
