Mollie is one of the horses and is very vain and prideful. The narrative also declares her to be stupid, and she prefers to be taken care of rather than working on her own. She is characterized by her love of ribbons, showing that she wants to be admired for her physical appearance. The ribbons show up when Mollie is mentioned; they show how she is concerned only with herself, and uncaring about the troubles or concerns of other animals.
...the others found that [Mollie] had remained behind in the best bedroom. She had taken a piece of blue ribbon from Mrs. Jones's dressing-table, and was holding it against her shoulder and admiring herself in the glass in a very foolish manner.
(Orwell, Animal Farm, msxnet.org)
Mollie is concerned mostly with her own well-being, to the point of stealing sugar and ribbons even after ribbons are outlawed. She also is unable to learn the alphabet, instead only learning to identify the letters in her own name; this shows her vanity. Mollie is lazy and unwilling to work as hard as the other animals, but in the end, she is one of the only animals who escapes the tyranny of the Animal Farm.