There were a couple of areas on the Batman gnome where you'll really want the Sculpey to blend in seamlessly with the ceramic. So you can mix up some Bondo to fill in the gaps.
Bondo is a two-part body filler material used in the auto body industry. It's like spackle but is really hard and durable when dry. Read the directions on the can. Mix it up in a plastic cup per the Bondo instructions. Then use a popsicle stick to apply it to the Batman ears and Batman mask where the Sculpey meets the ceramic. Let this dry a few hours. Drying Bondo smells great; it reminds me of being a kid when my older brother would work on cars. But you should probably head the warning on the can and ventilate.