Mollie, the horse who pulled Farmer Jones' cart, loves the pretty ribbons braided into her mane and tail and the sugar cubes her human masters feed her. She also doesn't like to work very hard. When the Rebellion comes, she doesn't find any pleasure in giving up her ribbons and her sugar or working hard for the cause of Animalism. She can't quite comprehend the principles of Animalism or why it is better to sacrifice a few material goods than to be enslaved. She doesn't understand that the ribbons symbolize the fact that she has been mastered, nor does she understand that she is being bought with a few trivial treats of sugar. None of this seems demeaning to her.
Mollie flees Animal Farm to return to her human masters because they offer her an easier life. She loses the opportunity to be part of building a society independent of the masters. One may argue that she doesn't lose much, however, because the pigs betray the other animals as much or more than the human masters did. Mollie is arguably better off—or as well off—with the Joneses than she would have been on Animal Farm. At least with the humans, she gets some sugar for her efforts.
Addressing the question about materialism, Mollie's example shows the power of materialism over ideology. Animals (and humans) love their creature comforts and are sometimes willing to give up their freedoms to keep them.
In this book, Mollie chooses to pursue material things. She decides that the most important thing to her is the ability to have cute things (like ribbons) to wear. When she does this, she loses her status as a member of the animals' society.
When it is found out that Mollie is continuing to pursue these decorations that are really something that "natural" horses don't wear, the animals think that she is too pro-human. They put pressure on her and she decides that she will be better off leaving and going back to having a human master.