My first thought was that it looks out of character, but I like to think of it as an evolution of Triss, in the same way that choosing Triss (if you do/did) can be seen as an evolution of Geralt. I think that giving her a sexy venomous looking outfit makes her look more like the powerful sorceress she is and is recognition/reflection of the fact that for many of us Triss was the ultimate romance for Geralt. She needed a look to elevate her to that part. Early game, I agree that it doesn't work when she's hiding out in Novigrad, but to me it makes perfect sense late game. I think it would've been cool if the game automatically switched her outfit once she left Novigrad, and she could've explained the change in the context of the situation or surprised you with it in cutscenes/dialogue.