Not sure why people think that Gojou is gonna crossdress when the classes can only have one contestant, and he would also undoubtedly hate doing it - and this manga is pretty good about supporting people with what they want to do, but not making them do what *you* want to do. It's about celebrating people's differences, so I doubt they'd force him into a cosplay if he didn't want it.
Also, in regard to Marin "outing" him to the class - I don't think Gojou has ever told Marin that him making clothes is a secret. He's totally fine whenever the two of them are talking to other cosplayers about it, and he works on his hina dolls and cosplay stuff all the time. She has no idea about his trauma, since he's never once brought it up to her or indicated that he doesn't want people to know. She also thinks incredibly highly of Gojou and his work, and no doubt thinks he's proud of it too, since she can't imagine him not being so. So I doubt she has any idea this might be something he doesn't want all of their classmates to know. There is something to say in that she should be more aware of his comfort level with her drawing attention to him, but other than that I don't think she did anything wrong based on what she knows.
I also don't think she's taking advantage of him and making him do all the work - she works very hard in her own ways (and while I'd like to see her learn more about how to make cosplay as well, that's besides the issue) and I think here she's simply going with the mood and being excited about things with friends, which includes Gojou.