Can you put sheetrock mud over wallpaper?

If you are speaking of very old wallpaper that is in fact paper, you might get away with it.

The difficulty come with the adhesive used to put the wallpaper up... most of it is water based, which means when you saturate the paper the adhesive lets go, and the whole thing has the potential to bubble, wrinkle, or in some cases fall right off the wall.

If the paper is newer, it may be vinyl or vinyl coated, in which case the mud won't adhere well at all.

My best suggestion is to strip off the paper, before applying the mud.