Appreciate all the positive responses and support, it's kind of you all. I do want to touch on the cosplay aspect to clarify the issue I have with it and will add to the original post later for future readers to make it more clear.
I believe a cisgender woman can cosplay a trans character, and that there's little issue if it's respectful. I feel there's a significant difference between that and what we're discussing though. This isn't a trans character in the game with any given depth, it's an illustration in an advert whose sole purpose is to fetishize the body of a trans woman, where her transness is reduced in full to her genitals and a comparison is drawn to a beast. The only reason it was of any note was because of the controversy surrounding it, and the issues trans people had with it. So I find it suspect as a choice to begin with, but there's still scope for it to be represented. What solidifies the problem is when the cisgender woman in question is treating that aspect of the trans woman as a joke; as something to laugh at. There's no desire to become any character, but instead become a walking mockery of a trans person that was only ever there to begin with as a fetishization. It's not as though cisgender women are de-facto in support of the rights of trans women, so when someone is treating our bodies as something to be laughed over, and is roleplaying us purely to serve for comedic purposes, I find it poor – to put it mildly. It further reinforces us as a joke, and that in itself is transphobic.
It also loops back to the advert itself. If it's designed to be a terrible and distasteful advert working at the expense of trans women – something CDPR themselves have told us we should be fighting against – then you can't use it frivolously for laughs or promotional material outside of that alleged context. Promoting someone taking that "terrible" imagery and further accenting and highlighting the parts we're supposed to take issue with renders that message meaningless.
If we were discussing a cisgender woman roleplaying a well represented trans character because of their love of them; that's fine. This isn't that though.
Furthermore there's been discussion around men cosplaying/roleplaying as trans women, which is an issue. The short version is that one of the largest battles trans women face in recognition, respect and dignity is in fighting against the perception that we're just men dressing up as and/or pretending to be women. So in as much as in a utopian society it would be nice for everyone to be able to "pretend" to be anyone else, that's not the world we live in. Men dressing up as trans women only serves to further embed in people's minds that trans women are characters being played by men, as opposed to women in our own right. Jen Richards puts it more eloquently and tidily than I could hope to though, using Eddie's Redmayne's performance in The Danish Girl as an example:
You could suggest that people understand the difference between a man pretending to be a trans woman and trans women themselves, but that isn't grounded in the reality in which we live. If you appreciate that this is something trans women have to persistently push back against purely as a means to live as (and therefore be seen and respected as) ourselves, then the concern around men pretending to be trans women should be understandable.