Filler doesn't mean pointless. filler just means it doesn't have to do with the overall story arc, and it COULD be skipped and you wouldn't be lost or confused.
However, that doesn't mean it's pointless. For instance, this book was very character-driven and we got to understand some of the characters a bit better. Rachel, of course, as well as Chapman. It was also the very beginnings of the Tobias/Rachel 'relationship'.
So no, it doesn't have any grand battle against the Yeerks, a new Andalite character, or a new Visser being introduced. Nothing that happens in it really has any impact on the overall story arc of the series, but it does have lots of characterization, which is every bit as important as any of those other things.