I don’t see any onscreen evidence to suggest that. She’s just one of many scraping out a meager living as a scavenger, and Plutt is the guy everybody sells their stuff to. (He may be the only person to do such business with.) The implication is there that she hasn’t taken any opportunity to get off-world before because of her forlorn hope her parents were coming back. It’s definitely not an Anakin/Watto situation.

If she was a slave, Plutt would have ordered her to hand over BB-8, not tempt her with bigger food rations.

Why she was left as a child with such a unsavory character we may never find out.