Yeah, quite a few Pokemon in the past have quickly fizzled out after having nice debuts...I can think of Noctowl as the best example, given its debut episode had it pull of some clever little plans and the like, but after that it was pretty worthless. But I'm not too worried about Miju...he'll be treated better. He's a starter.
Honestly I'm more concerned about Kibago. He really hasn't done any standout thing yet and just seems there, like, a blank slate. Yanappu, the "third mascot", or so it seems, from the previews shows more of a personality than it does, which is kind of disappointing as Kibago is a cool Pokemon it itself.
Oh, and Mamepato and Koromori also seem like blank slates at this point, but that's not too surprising given their roles and little screen time so far.