Mollie doesn't fit in this new society because she has absolutely no interest in it. It's all the same to her whether the animals are free or not.Ideals and principles mean nothing to her.All that matters to her is having nice things to eat and pretty ribbons to wear in her mane. In short, she is shallow and materialistic. In fact she prefers the old life when there were humans to pet her and feed her sugar lumps, and these things are not available in Animal Farm. She also baulks at the prospect of having to work hard. All she wants is to laze around and have a good time.
It is not surprising, therefore, when she ends up leaving the farm, secretly, to return to humans where she can once more have the luxuries she covets. The other animals are dismayed, if not over-surprised, at her departure:
None of the animals ever mentioned Mollie again. (chapter 5)
The animals, therefore, deal with her betrayal by simply ignoring it. At this stage, though, most of them do not realise that far worse is to befall their new, hard-won society than the desertion by a single, foolish individual.