Yes, I believe Cersei sent Doran someone else's skull. Either a disproportionately large skull from one of the dwarves brought to her that was mistaken for Tyrion, or simply a large man's skull, like Smalljon Umber, who was beheaded at the Red Wedding. Ser Robert Strong's face may be horribly disfigured from Oberyn's poison or they're just hiding his appearance but I think it's still the Mountain's brain under there.