Yes. Many of our customers have painted with clay paint directly over wallpaper. Usually, three or four coats are needed to cover patterns.
First, test on a small area to see how the paint adheres and covers the wallpaper's color and patterns. Also, check that the wallpaper is firmly attached to the wall. Clay paint contains a lot of water, and moisture can loosen wallpaper that is not securely attached. Pay particular attention to this with strippable wallpapers.
You can create a more uniform surface by applying lining paper over the wallpaper. It is an affordable option that is easy to attach and paint.