Trans person here.
I both agree and disagree.
It doesn't ruin immersion but it does remind me of something I'd rather forget about while I'm trying to have fun. I don't want to think about this shit while I'm playing video games, or at all for that matter. If the character wasn't there then nobody would have worried and nobody was exactly clamouring for a transgender NPC in an MMO before this. I can see it now, "World of Warcraft criticised for not having better representation like Guild Wars 2".
I don't like this stuff being shoehorned in where it has no relevance. By all means, include this stuff where it matters, or where it can make a difference, but in a game mostly populated by people who don't really care one way or another, or people who react the way that comments further down did, why? Why do it? It's not going to change anything, it's going to cause issues, it's going to cause arguments, it's going to cause insults. I don't need that, and nobody else does either. Save it for TV and movies where a wider, larger, more diverse audience can see it and be affected by it.
And even if it's relevant to the video game, why not make the character much more prominent? Why not make them important? They're not making a statement with this character, she's an extremely minor NPC. That isn't a statement, it smacks of nothing but shoehorning in to get some karma for representation without risking alienating their core demographic. Just my opinion, but it's like this is going to make a difference, just a couple of lines of throwaway, optional, easily missable dialogue.
ON THE OTHER HAND, it's nice to see that characters can be more diverse than the usual two-dimensional cutouts that NPCs are usually built to. Some would say a little diversity goes a long way. One of the relatively minor characters in Dragon Age: Inquisition is transgender, and the character is extremely well written. It was nice to see. And it was played as a cultural thing with the characters race, but it was still there and still well done.
The thing about representation is that it's representative. If you include a bunch of minority races, genders, sexualities, etc, then you upset the balance. The world, like it or not, is mostly straight and cisgender (read, not trans). Most media, therefore, ought to be mostly straight and cisgender.
Just my thoughts on it. Voicing this on Tumblr got me death threats (lol) in the past, hopefully it'll fare better here.
TL;DR: Diversity is important, but not diversity for diversity's sake. Turned out longer than I thought. I can go for hours about this shit.