Ah! The usual sea serpent.

Is XSLT good for transforming a [tree] data structure? H.ll yes !

Can it be adapted to transform JSON trees too? Yes, but it is a hack

Is it more declarative than coding mutations in a store? Yes.

Is it adapted to partial data chunks retrieval, aggregation and processing [also known as Web 2.0 or XmlHttpRequest or AJAX]? No.