There's something about this conversation in particular.... on a surface level it's some silly, light-hearted Mizuki and Ena banter. But the importance of outfits and dressing up is so deeply rooted into Mizuki's character and storyline, particularly when it comes to her desire to be understood. (Long essay post under the cut.)
This is comes up in a number of different ways (Mizuki’s love of fashion, her sister, the ribbon motif etc), but most relevant to this post is how clothing is brought up constantly during the Kamikou school festival events. Take the conversations Mizuki and An have during the first school festival event (Kamikou Fes):
Despite An's well-intentioned, but backhanded reassurance (I believe @ butterfly-ribbon had an analysis about this but can no longer find it since her account has since been deactivated. if anyone can find it please let me know and I will link it!) that "it's gonna be hard to stand out" at the festival, the fact that Mizuki is wearing her regular uniform and not the shirt DOES make her stand out. Despite being a student of the school, she's still an outsider -- but it is what it is, isn't it? Mizuki has resigned herself to that fate. Because the world has taught her that this is the norm, as someone who has to face constant harassment for being transfem.
Except, that doesn’t have to be the case.
The first Kamikou Fes event ends with Mizuki ultimately hanging out with the other named Kamikou students, and agreeing to wear the class shirt. Though she still hasn’t closed the distance with her own music group, she’s starting to understand that there are spaces and people who will accept her. She won't be on the outskirts of society forever.
Whither This Path of Thorns picks up with her slow integration back into "regular" school life. Note that Mizuki is wearing the class shirt from the very start this time; this is a direct way to signal that she's a part of the class! She's not a complete outsider, not anymore.
In contrast, Ena makes a huge fuss over wearing her own class festival shirt, and how much she dislikes it. It's played off mostly as a joke, with Mizuki teasing Ena about it, but there's this transparent desire of her not wanting to wear it, and thus Ena changes out of it the first chance she gets at the end of the festival.
So in this card story, where N25 changes into different outfits as one of the festival attractions, Ena is happy to wear something other than the t-shirt. In particular, she thinks she was especially cute in the maid dress, but Mizuki describes it as "cheating" and "not natural". As a defense against Mizuki's teasing, Ena points out that this festival is a time to "dress up and have fun". Yes, maybe the maid outfit isn't "natural", but who cares so long as it looks good, right? The whole point is to wear cute clothes.
The core of this exchange is extremely familiar to Mizuki, whether she directly realizes it*. The N25 main story contains a direct example:
*Though, she probably does. Mizuki has proven again and again to be self-aware and genre-savvy, but this is a slight tangent.
Though not perfectly identical situations, this is the same idea as before: "Super cute clothes! But, kind of weird, isn't it?" What Mizuki says to Ena lacks the transphobic context, but the exchange follows a similar pattern. Society will judge Mizuki for the way she presents and the clothes that she wears, even though it's what she feels the most comfortable in. People will admire (or, more accurately, gawk) at her from afar, but their gaze comes from a place of invasive curiosity about her "weirdness," and not genuine interest in knowing about her as a real person. This is what the maid dress is getting at, the idea of Ena temporarily being in a position where her outfit is the reason for her attention as opposed to herself as a whole. It's a small moment of her experiencing the world in a way like Mizuki does, which a minor running theme in Path of Thorns.
Ena's trained card from Path of Thorns also visually represents this idea of Ena being placed in Mizuki's position. Compare it to Mizuki's initial 3* card, as below.
Ena is posed almost exactly like Mizuki, laying down on her side and tenderly reaching for something beneath the surface (which happens to be Mizuki's "real self", though Ena cannot see her yet).
All of this culminates in that infamous line from chapter 7, where Ena's identity is questioned by one of the other students due to her association with Mizuki.
To be clear: I am not arguing that these situations are perfectly one to one. They certainly aren't. As a cis girl, Ena is never going to be a primary target of transphobia and affected by it in the way Mizuki is. Additionally, the previously discussed remark that Mizuki makes about the maid dress is not her lashing out either; in context, it is meant to be lighthearted and teasing in tone. However, a primary theme that this event and Mizuki's storyline as a whole plays with is the idea of being understood.
This is also why during chapter 7 and 8 of Path of Thorns, the difference in Mizuki and Ena's outfits is relevant. Ena is back in her proper school uniform, complete with her blazer, while Mizuki still wears her class festival t-shirt.
Without all her extra layers present in her school uniform, Mizuki is a lot more vulnerable than Ena, and the divide in their respective positions is emphasized by the difference in their clothes. No matter how much compassion Ena has, she still hasn't fully comprehended the extent of Mizuki's pain and fear.
So to fully understand how the other feels, Ena has to figuratively (sometimes literally) be put in Mizuki's position. When Ena is directly subject to the same microaggressions and comments that Mizuki faces, albeit on a smaller and less personal scale, it allows her to build empathy and complete a picture of why Mizuki feels the way she does.
Ena herself reflects on this after the rooftop conversation. She only now sees the harassment that Mizuki faces and how this has been weighing on her for so long.
It's not the single reason why Mizuki is so afraid of coming out to her group, but it is certainly one part of her fear: won't they also become a target by extension? Won't they also suffer? Mizuki directly expresses some of these worries herself in Wounded Hands.
The key thing is what Ena says in response. She'll let herself get hurt as collateral, because the alternative is to let Mizuki suffer alone in a world that hates her and never see her again. To her, that is the worst possible outcome. Arriving at this conclusion requires Ena to first understand the depths of Mizuki's situation, which is something she reaches only by being brought as close as possible to being literally in Mizuki's position.