If you used the "Create decal" action with correct parameters it should work.
You need to provide a UID of a Baby3D mesh first which is the one the decal will be applied to. Then select a tiled background object that serves as decal texture.
X, Y, Z are usually used with expressions from picking actions.
What's definitely not working properly are the normals in many cases which is why the decal may appear smeared at certain angles. It's on the todo list.