I think I had an epiphany about what's happening here. At first I was like what? That makes no sense! Just like everyone else, but then I thought what if the dialog was the villager saying "just call me Aquabro!"
I'm pretty sure I've had viagers say things like that to me before. Like look what I'm doing I'm gonna be so awesome at it you'll have to call me something pertaining to it!
Because it doesn't make sense that they'd call the mayor Aquabro if the villager was the one fishing either.