The species homo aquaticus sapiens, commonly known as Merfolk, are noted for possessing an unusual combination of qualities belonging to both Mammals and Fish. Most significantly is the addition of gills to permit underwater survival. They are also possessed of scales and fins for locomotion, and in most cases lack bipedal legs in favor of a single tail, though vestigial human leg bones are occasionally noticed in the anatomy. The humanoid with two fishtails in the place of legs is a variant called the Triton, and much less is known about that species.
Despite the fish attributes, the Merperson is defined as a mammal for possessing hair, distinctive bones in the ear shared by all mammalian species, and a mammalian reproductive cycle (as opposed to the laying of egg masses for outside fertilization), which includes the nursing of young. One need only look at the mastheads of the ships in our harbor to see know what every lonesome sailor dreams about.
Merfolk live in varied societies, like the human species. There are hunter-gatherer tribes, warrior clans, farming cultures (of seaweed and stationary sea animals as well as herding of fish), and entire cities (e.g., Atlantis, Mu). While there may be adornment of shells and such for ceremonies, and wear jewelry for ornamentation and indication of social status, Merfolk do not wear clothes as such. Their bodies are adapted to the temperature ranges of the ocean within their pressure range. The is no underwater equivalent of cotton or linen, nor a source of "wet wool". The hides of certain sea creatures may be used for securing personal equipment such as weapons.
The physics at the heart of the issue are that wearing of large articles of clothing will interfere with the process of locomotion and make the Merperson vulnerable to predators. Furthermore, because the breasts of mermaids are always exposed there is no taboo among their own kind about having them in open view.
My conclusion: Forcing a mermaid to wear a top is as illogical as asking her to wear pants. On a side note, if you want to try and start levying fines for sailors who don't cover up their mastheads, go ahead. I'll pick up your body on the dock in the morning.
Dr. Darien James Mason,
Licenced Av.Mdc, Av.Srgc
Miskatonic University, Class of 1828