Not sure what you mean by "stick it on". Fiberglass tuns often need support underneath, which is usually mortar or similar. But that is to mold to the bottom of the tub and provide support at the bottom, not necessarily adhesion. And I see no reason not to do it backerboard. I would use a skim coat of drywall compound to take off the roughness.
Always follow the manufacturers directions!!!
Always follow the manufacturers directions!!!
