Can I wallpaper over a decorative wall board or better to paint?

better to paint paper is not very good on wallboards

You can wallpaper if you prime with a stain blocking primer. It's easier to prime and paint.

Paint, you'll be happier with result

if the wallboard has any texture then paint is the way to go. There are liners that are made to be installed like wallpaper the will mask much of the texture so you can add decorative wallpaper or paint. Textured paintable wallpaper is another solution. It comes in many designs.

You can add fabric. Just soak it in liquid starch and put it on like wallpaper. It will come right off when it gets dirty and you can throw it in the wash and reinstall it. You can also change fabrics for the holidays. It will not hurt the wood .