You haven't directly said it, but it's the message I've got from your posts.
It is a broad book, it doesn't go into the depths of disability, race or gender. It's a general children's book about loving yourself. There are circumstances girls feel they cannot love their body, forced female circumcision, period poverty, abuse, the list goes on. There are circumstances black or brown people feel unable to love their body, physical or verbal abuse putting them down, perhaps causing physical pain. There are circumstances disabled people feel unable to love their bodies as you have articulated. These can cross over, you could have a black disabled girl who is suffering from every angle. You could also have a black disabled girl whose experience is completely different and is much more inclined to love her body.
This book cannot go in depth on these issues, it'd be too long and confusing for the majority of small children. It's a book which lightly touches over and sends a simple message of inclusion and loving the body your in, I suspect this is a lot more relatable to the majority of children in a UK classroom which as far as I understand this book is presently just sold in the UK. If you want a book that specifically goes into these issues, then no I can't recommend it for that purpose. But I'm struggling to see how anyone can reasonably criticise it for not being the book you want it to be. Do you get so geared up on everything that includes a disability that doesn't match your experience and question what the point of it being there is?
If a child doesn't want to join in that specific way, fine. However if it's because they're trying to will themselves into being different I think that's unhealthy. Your body is your body, it can't change and I think accepting that is important, and embracing that you might not be able to do things the same as your peers. But finding your own way to join in and do things and finding a beauty in that I think is a good message.