Can I use mortar to roughly level a basement floor for workshop?
Mortar will most likely crumble.

To ensure good adhesion, you could cover the existing floor with PVA before mixing up some granite chips with a little sand and plenty of cement. Leave it a little low, and don't bother where you need only a thin covering, then you can cover the whole floor with a thin self-levelling screed to give you a smooth finish all over.