I remember all the Barbie movies back then as a kid,they exist in movies and stories but real life I don't think so.
Well for starters, there's no scientific evidence to actually suggest the existence of mermaids, no half man half fish skeletons have been found and no lost underwater civilizations have been discovered...at least to the best of my knowledge.
The very concept of mermaids has mostly if not completely been based on lore and fairytales, there isn't a single shred of evidence to suggest that they actually exist, and even if they did, why would they go to such great lengths to keep themselves hidden.
Personally I don't believe that they exist, but I'm open to the possibility of being proven wrong. The oceans of the world are wide and vast and humanity hasn't even explored up to half of it. It's possible that they exist, but I'm sure they'll look nothing like what mainstream movies and fairytales have made them out to be.
I hope this helps.