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.