She is dumb unlike her sisters (friends) she's like a young child. I wouldn't be surprised if she act like that because there is no butterfly under the sea.
Why you conflating autistic with dumb? Also she's a fucking mermaid, grew up under the sea and all that? She had never seen a butterfly until recently so of course it excites her. She's also not human so her behavior is probably "adult" for a mermaid, but not meshing well with human society cuz she's not human.
She's not a kid, nor do her actions show her to be like one. It shows her entirely different upbringing and environment and culture. We can't judge her actions based on a human when she is just learning how humans act and doesn't know what people are really saying to her.
"butterfly, wait for me" omg author pleaaaase, she sounds autistic don't make her dumb please, she's an adult ffs