The others at least have some chemistry and interaction. What do these two do besides just stand next to each other at the wedding and sit with Eudora at the opening of Tiana's restaurant? We don't see them interact with one another.

I am neutral towards the other two, while I dislike this relationship.

What the heck do we even know about them?

They're just minor characters.

I don't like Melody, and we know extremely little about the Merboy from their interaction.

Even though Melody and Alex (the merboy) interact very little, I love the the interacts, they're very sweet and to me look a lot like a first crush.
I also find Mei and Yao adorable!
As for Naveen's parents...they don't interest me.

Because they are so young and not really together during the movie (they're just interested in each other) I don't consider them a couple.

I love Mei and Yao. I think they are a cute couple and I like their relationship.

Even though we don't see much of them in the movie and they are minor characters, we know what Naveen says about them and his childhood. I definitely like them more as a couple compared to Melody and the Merboy.

they are not a couple, it looks more like a sweet friendship. Even though we don't know much about Naveen's parents, they are officially married and have spent considerable time together and looked happy.

They aren't even a real couple!

There's nothing really wrong with them but the others interact and are both very sweet together.