An important preface to this review: I am autistic myself.
"Ableist trash" strikes me as a generous description of this film. The title character is more a collection of problematic stereotypes than a compelling character. Maddie Ziegler's portrayal of Music is riddled with caricature and exaggeration, and it makes an outright mockery of autistic mannerisms and body language. This is to be expected when you cast a neurotypical actor as an autistic character and expect said neurotypical actor to learn how to "be autistic" by watching videos of autistic meltdowns posted without the consent of the autistic people involved.
What's more, the movie isn't even about Music and the ways in which she navigates a neurotypical world - it's more about the neurotypical people around her and the ways in which they try to force her to fit their own lives. Hardly the "love letter to cargeivers and to the autism community" that Sia dares to claim this thing is.
Additionally, the musical/dance sequences that are ostensibly there to show the audience the inner workings of Music's mind are full of flashing lights and shaky camera work - both things can easily induce seizures in people with epilepsy, and approximately 1 in 4 autistic people also have epilepsy.
Sia's "research" on autism for this movie clearly leaned heavily on input from groups like Autism Speaks. A$ in particular is widely regarded as a eugenicist hate group among the autistic community, as it pushes for things like ABA (to force autistic people to mask and assimilate into neurotypical norms) and finding a "cure" for autism.
In short, this "love letter to... the autism community" reads more as a tone-deaf vanity project that spectacularly fails to give a meaningful representation of *any* autistic experience.