Not your target audience

Obviously the mouth depends on the kind of fish and the size of the reverse mermaid(i like the ides thst they come in as many sizes as aquatic creaturs do--some mermaids are teeny tiny, some are bigger than humans and not just because of tail length), but I think if they're human sized the mouth might be a good fit for like a giant or something else large(or with large genitals or nipples. . .oough hyper cock. . . .) Like they'd be prized by giants for being pretty good sex toys right? Their junk's too small to be played with(unless we're playing by gape kink rules or you really wanna split them in half or just rub thrm) but their mouths are a good portion of their head. . .and the lips are well shaped. . .always making that sucking motion, wouldn't it feel good?

They'd be pretty good for vore too, especially soft vore. Some fish'll try and swallow damn near anything, and if they enthusiastically try and swallow a friend. . . .

Of course that's just their mouths. Assuming legs includes like hips and below, I can imagine people 'riding' them while they swim. . .inside its hole or, if it's got claspers or an ovipositor or a dick or whatever else, with it inside them, hanging on as it uses its fins and body to swim around. . .kind of like aquatic belly-riding? Oh and of course foot and thigh jobs. . . .

I'm sure merfolk society would depend on which fish they are and what types of water they live in and all but maybe it's a merfolk society is like dolphin society where sex slavery is just kind of the norm. And if you have a fish-top kid you're in luck! Those're valuable because maybe their upper half is too slow or too dumb for them not to just be sex pets in their society. Holes or cocks just dangling out of their big heavy fish upper half, defenseless aside from the legs which you can just hold onto. . .whether the family keeps it themself or they sell/trade/lend them off to others, they're basicslly permanently in 'stuck in a wall' mode right? Their hips and legs probably aren't the best for swimming and turning with, and unless they have merfolk level intelligence they can't exactly protest. . .not that protesting has ever done anything in dolphin society, so in this case it wouldn't do anything here lol

Aside from being living glory holes they're also great breeding stock. Unlike other merfolk they can't really get away or fight back easily, as long as you keep them alive you can impregnate/get impregnated by one over and over. . .which again makes them valuable. . . .

On land it's a similar situation. . .even if they can breathe and even if they have strong legs, maybe their bodies don't hold themselves up well. Just poorly weight balanced. So more often than not they end up face down ass up, or stuck on their backs with no way to recover. . .at the mercy of anyone who happens upon their body. . . .

Of course this is assuming "lack of intllect" or societal(or individual--I'm sure some sailors would fish one up and keep it to use) cruelty. . .personhood and like ability to do certain things, maybe the upper fins function more like arms on an idk magical level. . .but even if they didn't, if they're able to advocate for themselves or someone advocates for them and they're able to live out of water. . .maybe there'd still be a lot of abuse and crelty for a long time haha but maybe things are changing for them, they're being heard, they're getting rights, being taken seriously, being seen as more than just convenient holes, as bodies to be sold by families or just thrown away or back into water. . . .