>>1630
What you must realize is that mermaids exist separately from monster girl culture. Sure, they're present in most monster girl media, but they also have their own media. Most other monster girls, i.e. lamias and harpies, are moe-ized versions of their mythological counterparts. Mermaids have always been pretty women with animal parts. While some of their oldest myths have them as seductresses who lead sailors to their doom (though the Greek sirens were bird-women), they've been sanitized since at least the turn of the 20th century. Having existed as relatively benevolent creatures in the Western subconscious for so long has caused them to become a cutesy but slightly risqué outfit you can put girls in, somewhere between a ball gown and a bunny suit.