Barbie and the Secret Door

Fantasy musical film Barbie and the Secret Door may not be one of the mermaid-focused Barbie films, but mermaids do make an appearance in it.

In this movie, human princess Alexa discovers a magical door that leads her to a fantasy realm inhabited by fairies, mermaids and unicorns. One of the characters she befriends is Romy, a mermaid who had her tail taken away from her and is stuck with legs, much to Romy's distress. Romy was perfectly happy as a mermaid and doesn't seem to like legs because of how clumsy they are. Her speech is also very clearly mermaid-ified, what with her usage of "swim" instead of "run", and "dive" and instead of "hide". There are also a couple of other mermaids in the background of the main story, but they're only seen briefly.

Thanks to Siren for letting me know about this movie.