If "monster-girl" is made into its own category, it should be aliased with many more things.
"monster-girl"
"monster-girls"
"monstergirl"
"monstergirls"
"elf"
"elves"
"succubus"
"succubi"
"lamia"
"lamias"
"mermaid"
"mermaids"
"harpy"
"harpies"
"dryad"
"dryads"
"slimegirl"
"slimegirls"
"slime-girl"
"slime-girls"
"dragongirl"
"dragongirls"
"dragon-girl"
"dragon-girls"
"lizardgirl"
"lizardgirls"
"lizard-girl"
"lizard-girls"
I'd just hope they don't associate the whole thing with Furry/furries as a tag, lol.
That could cause a bit of an advertising/browsing uproar lol.
People purposefully tag their stuff as "monster-girl" to make a particular point that it's not "furry".
"Monster girls" implies to me monsters that are female, young-looking, and have either sexual or moe appeal. This can overlap with all of your list, but elves can be (and frequently are) male, harpies can be (and frequently are) old, and even obviously "sexy" monsters like mermaids and dryads can be presented as inhuman horrors.
Well, I am certain that there are folks out there who find male elves, old harpies, and C'thulu-esque mermaids/dryads to be attractive and/or adorable, but I'd agree with the sentiment that you were trying to make that "monster-girl" represents some specific visual style or at least a specific range of visual features.
I'm not trying to police anybody's interests or even enforce a specific style, but I do think that the monster-girls tag should be restricted to, well, girls (who are also monsters). Male elves should be tagged monster-boys (assuming that "elves" even qualify as "monsters"), old harpies should be tagged monster-old-women, and eldritch mermaids should just be tagged monsters.