Mollie is a lot like the character from Greek mythology, Narcissus. If you have ever heard someone described as narcissistic, it simply means he or she is in love with him or herself, self-absorbed, or vain. Mollie, like many people in this world, falls into this category. She is more concerned with maintaining her beauty with Mr. Jones ribbons, admiring her name written in sticks, and protecting herself as she did in the Battle of Cowshed, than making any positive contribution to the good of Animal Farm. Mollie, like many people of the world, is concerned only about herself, rather than the good of her community.
Mollie also loves the sugar that Mr. Jones would provide for her. This makes her loyal to him. It also proves that some animals received better treatment than others.
Sugar is something that tastes good for the here and now. It's long term effects are not productive. Sugar means a temporary good feeling that later leads to a crash. Sugar is fattening. A little is okay, a lot is problematic.
This is like the real world because people are so apt to seek things that immediately satisfy, not that which takes work. Those things that satisfy right away often aren't lasting. Those things we work for over a period of time and commitment prove more fruitful for we become better because of the journey.
If you think about how Mollie is, you will see that she is very vain and that all she cares about is herself and how she looks. She is willing to abandon the other animals all so that she can have ribbons for her mane and tail, right?
This is why she represents vain and selfish people. Orwell is saying that there are many people in the world who do not really care about being free or about helping those around them. People like this only care about whether they have enough material goods to keep them happy right now. They don't think about whether their lives could be better or whether they should aspire to something more than just having ribbons in their mane (Mollie) or a nice plasma TV (people).