If you are wondering, based on the title, if this book is culturally appropriating Japanese culture for some laughs, then yes you are right, they did a racism. Oh also everyone is white because of course they are.
Beyond the orientalism, which goes exactly where you think it will go (“Grandma made dinner instead. It was raw fish. It was horrible.”), Grandma tries to be a ninja. Pearls on her neck, chopsticks in her hair, carrying a handbag through the whole thing.
Hijinks ensue, because older women? Being active?! HA! What a ridiculously silly idea! (This is also sarcasm.)
Playing off the stereotype that older adults are by default weak, unable to learn new things, and grandmas are boring, this kid’s grandma learns karate and is good at it. We’re supposed to be surprised and amused by this.
But don’t let it get too far – Grandma is also not particularly bright, and she’s disruptive and kind of a burden to have around. Like, the makers couldn’t just have a laugh while denigrating older women and Japanese culture, they also had to add in another layer to remind us that even the best and most active older women are still kind of shitty at everything.