Moll doesn't really face one single antagonist throughout the book, but she does manage to get in a whole host of sticky situations, each with their own problematic characters. However, Moll tends to get past those hurdles quickly and moves on to the next adventure. No one person presents Moll with the kinds of threat we're looking for in an antagonist.
Instead, we might say her biggest adversary is traditional society. But wait, you say, can a society be an antagonist? The answer to that is a resounding absolutely. Why? Well, English society places Moll in positions she doesn't want to be in all the time. Plus, it has a bunch of different rules and regulations that limit her options and opportunities.
Moll does the best she can within those bounds, attempting to save money, move up in the world, and, when the conditions are right, find love, but she can only do so much. And of course what she does do ends up being either immoral or illegal most of the time. We can't help but thinking that if perhaps Moll had lived in our day and age, she might have been able to skip the whole life of crime and blazed a path to the top with a lot less pain and effort.