@Fairtale Before I respond, I'd recommend you to not watch 2.5 Dimensional Seduction which is also about cosplaying like My Dress Up Darling is here, the difference between the two is while one (Darling) has some ecchi to it, it's mostly handled better here (mind you both series have uncomfortable moments but I digress) than 2.5 handles it's content and moments. I think a lot of people are right to compare 2.5 to Darling and frown upon the choices made in 2.5 that can be scummy at times and nonsensical at others (E.g. some ecchi cliches are in 2.5 where as they aren't in Darling). But having said that, a LOT of people also feel Darling is scummy and that Marin is as you said which brings me to replying to your reply.
I cannot deny there likely is some lv of a checklist the author used when creating Marin. But if you stop and look at most anime, you'll see there's usually a checklist of things in any genre/theme/characters and such. Should we then hate on all anime and characters and be reviled by them for having those check lists? No. Look at Isekai as a GREAT example of this, its the kind of anime that Harem anime once was, over produced, to the point you have so many same things happening it gets old, fast and stale. And I love Isekai I should add but even I say it's time to move on from them. Harem, suffered the same, having it's own check list and same result, so many male leads who never settled on a single girl and usually bland, dull, boring, self insert character and then have 2-5, sometimes more girls fawning all over him, often for some of the most dumbest reasons imaginable. And ofc, theres the other check lists for them am not even going to mention because I'm getting off topic. Back on topic, the point am trying to get at is, in the case of oh this girl is covering a lot of check lists and there's a male lead (rarely a female lead but they exist both for female love interest and male love interest, Otome in the latter's case) she is or may be into, there's usually a check list for this. Heck, most anime girls especially these days, are designed to be Bishoujo (aka beautiful/pretty girls) because, sad reality is, beauty sells, if something isn't pretty/attractive it can turn people off from it which is why most shows (not all I will add) and more so these days than in the past feature girls and guys to a lesser extent, being pretty/handsome.
As a perfect example of this, I legit cannot take an anime girl seriously when she says she wishes she was pretty... like, girl, you're already pretty enough! Anyway, check lists for everything almost exist, it is how stories are created in any media, I can get why some would take issue with Marin (and more so her + the series' name even if I think it's a great and cleaver name) but I'm just saying, ost anime have that checklist for girls in them, in most shows, maybe not exactly like Marin, but it's still there to some extent. I choose however to focus on the positive sides of Darling and Gojou and Marin and co. It's LEAGUES better than 2,5 is which handles the topic of cosplaying in such a way that it straight up IS what you said Marin and Darling is. Where as Darling here tries to be respectful to the material, that is, cosplaying despite some of the checks on the list you mentioned. In fact, if you were to watch even the 1st episode or 3 of 2.5 I think you'd have a better appreciation for Darling and Marin than you do now. Please note am not saying you should watch 2.5. (Iirc I dropped 2.5 late into the show)
As to the last thing you mentioned, I do believe part of it is your perception of bunny girls and what you said, not all of it sure, but still you have a negative outlook towards bunny girls because of playboy. I can't really help you there but I can tell you if you google anime bunny girl, you'd likely find tons of anime with bunny girls in it or figures of them even if they didn't wear the outfit. I personally see nothing wrong with the outfit and my best advice is, to try not to let it bother you and separate the two.
As for her being "too perfect in an almost plastic way", well, keep in mind the story is still being told (and I don't read manga almost ever and haven't for this series) so maybe some faults will be shown later (or maybe they already were and I don't recall them because it's apparently been 3.5 years since S1 aired...) but even if not, I can understand where you're coming from but this isn't an issue I feel with this series much less with Marin. I'm a big Otaku and proud of it. I see myself a lot in Marin. Though she and I are very different girls. As for her showing too much nakedness. I do know you aren't recall the outfits she's worn from S1 and likely have forgotten she cosplays as Eroge characters, some of her outfits are just that, revealing and I personally don't see anything wrong with that but I get her not wanting to show too much skin such as with this bunny girl outfit, her solution was basically skin suit XD she can be a bit quirky at times but I love that side of her, dunno if it'd be a fault but adds to her charm and can be a bit weird at times where she draws and doesn't draw the line but nothing wrong with that. Anyway, point is, you shouldn't let the amount of skin shown bother you.
I think this ep bothering you isn't just a you thing though, in my case it bothered me but for different reasons being, I ain't comparing the art style but I don't recall it being this low quality that it's noticeable worse and the art style changes which I commented on were both taking me out of the experience and ep itself. Sure the animation is better I believe, but that's not always a good thing if the art style has to suffer for it. It was a largely good ep, with soem faults, such as what I mentioned but also the pacing was bad such as the jump to that party... hopefully future eps will resolve any issues ep 1 had.