There are a lot of people who aren't healed from their ex(es), and that's why they're either the person creating drama or the person always quick to frequently talk badly about them. They may no longer be in love or care about the ex(es), but they still feel a lot of anger, resentment, and shame for having been in toxic relationship(s). This is not healthy for the new/current person because it just means that they're being used as a distraction. If the new/current person is more attractive, then the other person is probably hoping it will be easier to help detach from their disappointment and shame.
When you see someone put in less effort with the new person who is more attractive than the ex, it means the new person is nothing but a void filler. If you're the new person and you know that you're more attractive and have a lot more to offer than their ex, but you don't feel that this person is putting effort with you like they did with the ex, then it means you need to start detaching. They are just using you to cope with their trauma.