Hiya!
It's not offensive to ask questions, don't worry. Effectively, White Day is traditionally a men's holiday and the inclusion of The Trans Girl Character is uh, an odd choice. (here's a source to explain more about White Day and the origins of it but you can find plenty more if you need to do some more research) https://tokyotreat.com/blog/march-14th-everything-you-need-to-know-about-white-day
Project Sekai definitely follows the traditional male white day due to the way they treat the holiday. The login bonuses uses art of the men (and as of this year, Mizuki...), the art they post on twitter for the holiday only uses the men, and they follow this with the gacha of it too. The first two white days covered 5 guys and Mizuki. The third White Day, when they basically ran out, morphed the appearances of the girls to resemble more traditionally masculine figures--it's particularly noticeable on the card art for Kanade and Mafuyu. The intention for these events is well, men/male fanservice, because White Day is a holiday centered around men...
The gacha is the most important part of any event, because those are the ones with the most work put in, have outfits, hairstyles,
So, why is it weird to include Mizuki?
It's because they clearly made it a priority to lump Mizuki in with the guys on a traditionally guy-holiday. Mizuki is in the first White Day event, after all. With Tsukasa and Akito. The second one brings Toya Rui and Kaito in. The intent is to bring as many of the guys in as fast as possible. It's to profit off of Mizuki "as a guy" to appeal to certain gross parts of the fanbase. To lump Mizuki in with men/masculinity.
That is the reason that these three characters are "associated" together, because of transphobic business appeals to male fan service.
The card itself is fine and so is the costume, Mizuki is handled respectfully in that front, it's the intention behind it's existence that is weird.
I'm sorry if this is a bit difficult to read/understand, if you have any additional questions please feel free to ask some more.