This is nonsense, and you only think this is the case because you were focusing on it so hard. Before we knew anything about her we saw that she was a bright, attractive, and canny kid who was interested in everything. Then we saw her shed and learned she made little stuffed creatures to sell for extra spending money, and we see she wants to help this weird creature rather than panicking and running as most people might. Later Donna says she does acting in the drama club but isn't very good at it. That's more than we learn about most secondary characters in a single episode.

I don't know you, so I don't know what your experience is and won't even venture to guess, but I can say from my own personal experience that people with trans or otherwise queer kids talk about it, especially with other parents. And it is sometimes quite a challenge for some, especially for older relatives to "get it", even when they want to do right by their family member.

No, it's not the only thing about Rose, but it is an important thing, especially when vast squelching chunks of society would rather not even acknowledge nonbinary and transgender people even exist. And I'm glad they were up front about it, rather than drawing it out and trying to be coy to the point of wanting to have it both ways.