Wow, I think you interpreted this as an attack... Really it's just my perspective. I totally think it's fine that others see it as more likely to be fairy. Really! By no means do you have to be convinced of my logic! My opinion is subjective! If I came across as harsh, I apologize, and if I misinterpreted some of your points, I also apologize.
Agility is definitely a general Pokémon thing, or rather, a thing about fighting in general, but I think it also pairs well with the fighting type and is seen in a lot of fighting types. You also have agility associated with a lot of flying types, but not, say, bugs. In my head, there really isn't a big stretch between a trained athletic disciplined fighter and an acrobat- especially if the circus theme grows into the strong man motif. I see acrobatics as complimentary and essential to many fighting styles like martial arts.
I really, really, really think it is strongly based on a clown. Not even saying it will stay a clown per say, but I think it will keep a circus motif. It has the big red nose, the frilled clown collar, it's snout is reminiscent of a clown hat and even the animal basis- seal- has strong circus performance associations in the exact way popplio is being presented. He's a performer, definitely, but I think he's a specifically a clown.
I know you said performer to performer, but that's too broad of a classification system in my eyes, and not typical of starter designs. In my opinion, they are more specific. Greninja started a ninja and stayed a ninja...he didn't start as a samurai, change to an assassin and then go ninja because they are all "rogues" or "fighters". Animals bases are often mixed, so I'm not arguing against it taking on another animals traits like Chespin does. I'm talking strictly design inspiration.
I also don't think that sirens, in most folk lore, are seen as "entertainers" and "performers", more so that they are seen as dangerous and predators that use their voice as a lure. They are captivating, but not for the purposes of our entertainment, not because they are fun and silly, and not because they want to perform. It's because they want to drown others. And I just don't think that fits Popplio as he's been shown.
But, I could be TOTALLY wrong,and if I am, I will be the first to admit it and voice my surprise. And again, I'm not trying to argue that fighting IS gonna happen or IS most likely, I just think this the most likely of the two. There is certainly room and reason for speculation on both sides
I'm not even saying it can't become fairy...which I can see why that would be confusing when I say disarming voice =/= fairy. I just think it LESS likely than fighting specifically. I certainly think between ice and fairy that fairy is more likely, for example. I also think a Selkie inspiration, regardless of typing, highly unlikely as well as a mermaid inspiration (but I do LOVE mermaids and have wanted a mermaid Pokémon).