Mollie never really believed in the revolution. She was not part of the Battle of Cowshed, she was frightened by the sound of the guns. Mollie also has not accepted the fact that under the new terms governing the farm after the successful takeover by the animals, that she must work.
Mollie was more of a pet of Farmer Jones, not interested in anything other than her sugar cubes and ribbons for her hair. Every chance she got, she avoided work, arrived late or finally, she is seen being stroked by a human. Shortly after that she disappears to be seen by the pigeons to be pulling a cart for another human.
Mollie's disappearance can be compared to the Russians who chose to escape the revolution, not believing in the potential of the totalitarian state to take care of their every need. Many Russian people left not expressing any interest, like Mollie, in the revolt which was intended to bring equality and justice to all people.
Mollie's actions are significant, since on Animal Farm, there really is no room for any animal to disagree with Napoleon, who is a representation of Josef Stalin. Her escape, like that of many Russian people was very brave, and courageous.