Yes, mollies are fin nippers. Despite being peaceful and friendly, mollies are notorious for being fin nippers. Male mollies especially love nipping the fins of other males to establish their dominance and defend their territory.
Mollie fish also tend to nip at other fish with large, flowing fins.
Unfortunately, there are no definite ways to stop your mollies from fin-biting. To mitigate the occurrence of fin-nipping, avoid keeping mollies with fish with big, flowy fins, such as angelfish, fancy goldfish, and gouramis.
Additionally, keeping your fish in a larger tank can prevent territorial disputes and thus, prevent fin-nipping.