What does that even mean?
What does it bring to a plotline to labour the point that a trans person is someone who's female, but who would like to be male, and so dresses in clothes that are stereotypically male, and has changed their name etc.
Mentioned in passing, totally normal part of a book. Labouring the point? Preaching, not normal in a book.
Someone being trans as the sole point of a book for 3 year olds? Nope - it's not part of their life experience yet - let 3 year olds be 3 year olds.
Someone being trans as the sole point of a book for 15 year olds - maybe, as long as it's not misleading/lying/encouraging harmful behaviour - just as I wouldn't want a book glorifying bulimia, or brushing over the difficulties of any other health issue if it was the key plotline of the book.
In the majority of books, the fashion choices and drug regimes of a character just aren't important - mentioned in passing to add flavour, but not key to the plotline, and that's the same whether it's them being a goth, trans identified, skater or diabetic.