So ignoring all the other hydrodynamic inefficiencies associated with sticking a human torso on a fish body, let's just worry about those most distinct of human female features, breasts. The problem with them on mermaids is that no other sea creature has breasts. Sea mammals such as whales and dolphins have teats that calves will suck from when nursing. But, these teats are flush with the skin of the mother and offer no additional resistance when swimming.
Assuming the instantaneous creation of classically shaped merfolk, would evolution select for or against human style breasts on an creature that lives exclusively underwater? If mermaid breasts did remain, how would they be shaped and where would they be located for maximal hydrodynamic efficiency as well as satisfying their role as providing food for merchildren?