I can 100% understand why the movie would have a special resonance for a trans viewer, though. Mulan has a lot to say about gender roles and what it's like to be forced to present yourself as something you aren't (going both ways, too -- Mulan is uncomfortable dressed up as the perfect bride, but also uncomfortable dressed up as a man.) But the letter definitely implies that it's wrong to see Mulan as cisgender, and that's clearly gross.

But also, the actual movie that really lends itself to trans interpretations is The Little Mermaid. If you really want a nonbinary Disney princess, Ariel would actually work super well.