The issue isn't making her seem like just just a fishy human.

The issue is one of tone, a character's backstory involving hundreds of child deaths really does not fit this RP tonally. On top of that, it doesn't really make any sense for Mermaids to have children like that. Presumably they take care of their children and don't have dozens of things eating them. If it was like that it's really unlikely child protective services wouldn't intervene with every birth.

If you want to make her more then just a fishy human, have her born from an egg. Have her be able to breath under water and still not be fully adjusted to being on land. Applying that trait to mermaids really doesn't make any sense and is extremely dark for this RP.