Well, here we have it, an anime that delves into the topic of cosplaying... or at least, that's what it was supposed to be. The reason I say that is because while yes, the characters interact with cosplaying and they talk about how they like to dress up as them, act as them, etc., all these start to take a back seat later on in the episodes. This point is further emphasized when you compare this anime to another otaku-based one: “Saekano.” “Saekano” was an anime that brought a perfect balance between the harem/romcom aspect and the otaku/game aspect. If one were to watch it just for the otaku/game aspect and not care about the harem/romcom, they would be satisfied. If it was reversed, they would also be satisfied. This cannot be said for “My Dress-Up Darling.” This is because after roughly 4-5 episodes, the whole cosplaying charm starts to feel more like an afterthought, and the only reason you would really want to keep investing in this series is for Marin... which I guarantee you, is the only reason why this anime got as popular as it did. "What do you mean by that?" you might be wondering. Well, that all resolves around the topic of one character; that character being... well, Marin.
Marin is the only reason why this anime became as successful as it did. Whether or not that is a good thing is up to you, but regardless of such, this does not bode well for the other main character of the story: Gojo. Marin completely overshadows Gojo and doesn't really give him time to breathe throughout the show. This creates an imbalance between the two as one is constantly being the star of the show (Marin) who is plastered onto everything that coincides with what defines a wish-fulfillment "waifu," while the other (Gojo) takes pretty much a back seat. As such, this results in the story being more of a fluff "how much can we make Marin cock-tease Gojo?" because, this is an ecchi rom/com after all. As one might expect, this starts to make the "comedy" too same-y or too repetitive because the audience always knows that Gojo isn't going to be the one to insinuate anything, it's Marin. Speaking of teasing, this is where Marin, as a character, starts to show how contrived and desperate she is to appeal to the audiences.
While many see her as "Aw, that's so adorable!" or "OMG! This is best girl!" this is where she falls apart. Marin appears to be too try-hard to appeal to the male otaku audience. Yes, it's as if the author sculpted this character to desperately check off every box on a list of what makes a male otaku teen horny in the most blatant way possible. Why don't we go through every quality she has to show you what I mean:
She is popular in school, likes to watch anime and cosplay, isn't afraid to show off her body (which goes back to how she cock-teases Gojo), likes to play porn/eroge games, and to top it all off, is extremely attractive both design wise and in the anime's canon.
In other words, there is nothing really natural about this character (yes, even by anime standards), which makes this series—depending on how you look at it—come off as very try-hard or, as I stated before, contrived. Marin just fills these criteria in such an over the top way that it makes it seem like the author was saying, "People of the otaku audience, please watch/read this! There is a character that will make you horny *wink* *wink*." Basically, the author rode the "Manic Pixie Dream Girl" trope way too hard that it becomes monotonous. What makes it worse is that the series is extremely reliant on making Marin attractive that yes, there is fan-service. No not the occasional "oh ha ha, he fell and saw her panties." This is more like almost an entire episode having Gojo look at Marin's tits or legs or whatever and getting turned on—all with obscure camera angles to emphasize Marin's attractiveness.
So yeah, those are the flaws of this anime. Now, you might be wondering if there is any good, and yes there is. The animation is really nice and consistent, the OST is good, the art is really good, and... yeah, that's about it. Everything else (e.g. comedy, characters, etc.) I already stated, so that's all there is to say about this anime. I forgot to mention that there really is no story. It's a slice-of-life type of anime, so if your looking for some really deep romance story for instance, look elsewhere.
