Some people do use mortar under a shower pan, but it's not always necessary. If your subfloor is sturdy and level, you may be able to get away with just installing the shower pan directly on top of it. However, if you're not sure about the condition of your subfloor or if you want to be extra cautious, using mortar can provide some extra stability and protection.

There are different ways to install a shower pan, so it's best to consult with a professional about your specific situation. Generally speaking, though, you will need some kind of adhesive or mortar to attach the shower pan to the subfloor.