This is one of the Standard /tg/ Responses. I see three that are often invoked whenever some classic fantastic creature is under discussion:
1. Make it realistic.
2. Make it evil, or at the very least monstrous.
3. Return to the “original” conception of the creature or entity— and bonus points if that original conception is more lethal than the current, popular interpretation.
This sort of attitude is most prevalent in discussions of the fae— on /tg/, Tinkerbell would apparently be a radical departure from the standard notion of a fairy— but it gets applied to just about anything under the sun, to one degree or another.
This is certainly a valid point of view. Personally, I like light-hearted adventure with a minimum of murder-raping and heroes who are actually decent people, and evil mermaids (or, e.g., evil unicorns) don’t fit that scheme too well. It could work if you turned them into major villains, but otherwise “sea-women who want to drown you” just seems less fun than “friendly sea-women who you can interact with in a way that does not involve violence.”
We now return you to your regularly-scheduled (every day— well, every other month or so— until you like it) thread about enormous mermaids.