I'm not quite sure how I would feel if someone told me they wanted to be my mermaid. I get that "The Little Mermaid" is back with a live action take on it. Mermaids have been thought of as these magical beings for such a long time, but I never really thought about them much. This song as also romantic undertones and I didn't really like the movie "The Shape of Water", so I don't think I can get into someone wanting to be my mermaid.
At the same time, I'm all for people expressing who they are and who they want to be so long as they're not harming anyone else. If Sia Shells wants to be a mermaid, then Sia Shells can be a mermaid. And while that may not be the type of scene I'm into, it is fair to say that the reverse of that is true and there are people out there who aren't into what I am into. So you just have to do the "different strokes for different folks" thing here.
Now the biggest thing about this song is the sound of it. It feels magical. It is pop and yet also feels like something a mermaid would make because the music has reminders of the ocean. It's catchy and if there is a niche for this then I definitely feel like it would fit into that crowd very well. In terms of music itself, this song is very well done.
Realistically, no matter how you feel about the content of this song in terms of the lyrics, the message is still there. This could just as easily be someone singing about wanting to be your boyfriend or girlfriend or whatever attracts you. I think that having an open mind and leaving the lyrics to interpretation works here because this isn't the first time such a concept exists (see: Aqua "Barbie Girl") but the music is just too chill to ignore.