Over my reread of the Animorphs series, my overall verdict has been that it's much better and more thematically mature than it's commonly given credit for. We get to see themes of trauma, slavery, ethics, the necessity of making horrible lose-lose choices, and overlying all of it, the horrors of war and the ways that it permanently alters people and tears families apart.
This book is more like a Saturday morning cartoon.
As a Saturday morning cartoon, it probably would've been fine, but it is nowhere near the standards that I've come to expect from this series. I mean, tiny aliens with outsized egos? Shrinking shenanigans? The series main villain (not to mention the series main plot) is only peripherally present? What was wrong with the alien invasion we already had? And why do the stupid, pointless filler books always seem to fall to Cassie?
I mean, I guess it is understandable that after the nonstop horror of the David trilogy, the Downer Ending of The Hork-Bajir Chronicles, and the sheer emotional intensity of The Pretender, the author might have wanted to give us a breather with a bit of comic relief. Just... did it have to have so little to do with anything that's even remotely related to Animorphs? And did it have to be so cartoonish?
As usual with mediocre filler books, the sole bright spot in this whole mess was Marco, and his constant stream of sarcastic riffing (especially his "groveling" act post capture by the Helmacrons) was worth the occasional laugh. Then again, it was all kinds of disconcerting to have Marco of all people constantly spouting impromptu physics lessons (isn't that Ax's job?) to explain things like their increases in strength after they were shrunk—yeah, Marco's smart, but he's always been more of a tech guy and a strategy guy than a physics nerd.
Overall? It's weak, and it's not worth the time.