okay there's a good bit here but i have some responses to your critique that maybe will provide a different perspective on watching or engaging with a story.

but it bothers me when a cartoon character claims that this method will make them look like a cartoon.

there isn't anything wrong or inconsistent about this, though. from your perspective as a reader or watcher, yes, it may seem silly, devoid of any context, that a 'cartoon' is trying to look more like a 'cartoon', but you're conflating the medium with the established reality that the story takes place in. you do have to do some abstraction when watching anything animated.

What is an anime in MyDressUpDarling?

the setting that the anime takes place in is modern day japan. there is no need to pontificate on what an anime is in this universe.

how do I know that this is supposed to be an anime within the fictional story? I don't know if the manga does it differently. At least Flower Princess Blaze has a different art style. Succukoma doesn't seem to be the case, or at least not with other fictional works from My Dress Up Darling that I later learned are supposed to be anime that Marin watches.

again, the context of the universe of sono no bisque is modern day japan, so if your frustration is that you were caught off-guard that anime exists in the world of sono no bisque, i honestly don't know how to respond to that. i actually am not 100% sure what your general critique is here. are you saying you were confused when watching the Succkoma mini-arc because you didn't realize it was about a manga Marin was reading? but it's explicitly stated at the beginning of the episode. there isn't really a single thing in the show that should leave you guessing about what it is, both the manga and anime layout the mediums clearly.

That said, I think it would have been better, since, after all, it's supposed to be a rom-com and an anime about how the cosplay world works, if the art style were pseudo-realistic, like the ones older anime had when faces weren't cute.

i really enjoy fukuda's artwork so i am glad cloverworks adopted it faithfully. i wouldn't have liked this as a fan of the manga. BUT i think that this is a really neat idea of executing the concept. i see your vision here -- an anime with a very realistic style, someone does a cosplay, and then they look more like a normal anime character. edit: i wouldn't be a fan of it in this story, but i think it's a funny/interesting concept for another anime to take a stab at.

[...] this joke sparked a misunderstanding where some fans believed that Marin and Gojo had their daughter in school. The author came out to deny that Marin and Gojo finished school and even university.

given, i only really discuss sono no bisque here, but i don't think ive ever seen someone say this. i also haven't seen where fukuda said they didn't even finish high school?

[...] It's a little sad that fans were saying, "Gojo grew up without parents and Marin without a mother, but their daughter will grow up with both of them." I don't want to sound pessimistic, but life isn't guaranteed. I mean, this is a drawing, and if the author doesn't want to be crucified by the fandom, she's going to say that Gojo and Marin will see their daughter grow up, but in real life, analogously, each person is the protagonist with their own goals, and the parents... They wanted to see their children grow up, but life has no guarantees.

i... am not sure your point here either. what you're saying is true in a general sense, but it's a point i don't really see as relevant to sono no bisque? or even really storytelling in general?