Mollie had never truly bought into the revolutionary ideals of Animal Farm. She sees the advantages that being owned by humans has over Animalism. In fact, she thrives on the attention of humans, enjoying her groomings and the way people tend to spoil her with affection and treats.
As a result, Mollie never really followed the new socialist rules of Animal Farm anyway. She is self-centered and vain, qualities that will not serve her well after the animals take control. After the humans are expelled, Mollie avoids her duties and secretly keeps sugar and ribbons to herself. All this quickly convinces Mollie that Animal Farm is not the right place for her. She leaves to once again put herself under the care of humans. She certainly seems happier after her departure. The pigeons report that she was seen in town pulling a small cart while being pampered by her human caretaker.
Mollie is meant to be symbolic of the Russian middle class. Like them, she benefits to a certain extent from the oppression of the rulers under the old system. She might have been sympathetic to the revolutionary cause, but she never really wanted to give up her comfortable bourgeois lifestyle and make sacrifices for the greater good of the collective. Considering what happens on Animal Farm, the reader is left to wonder if Mollie makes the right decision by leaving.