I do believe (although I may, of course be completely wrong) that gender disphoria is recognised as an illness by the Church to the extent that a person with ambiguous gender who was registered as and self-identified for many years as a male and had surgery to become apparently female was eventually admitted to a convent and lives as a nun with no issues whatsoever.
I seem to remember hearing that account from two different sources, although I can’t give any references.
So, to the extent that cross dressing is a symptom of a deeper malaise afflicting a person, then it may not necessarily be sinful.
As ever, for a sinful state to exist, it has to be a wilful act with the knowledge of the gravity of the matter, BUT that there may well be extenuating circumstances.
St Paul decries those who swap their natural state for that which is unnatural, but I’m pretty sure that had such advanced medical options as we have now been available, he wouldn’t have condemned those who did ‘cross dress’ because their expected outward appearances didn’t match the struggles they had with their inner condition.
I don’t think there are many natural and healthy men out there who feel it necessary to pass themselves off as women just because they can. There has to be a deeper reason.