I'd attribute the look and quality of "adult cartoons" to a lack of budget. There's a reason a lot of them are dysfunctional family comedies set in mostly realistic environments: It's the cheapest thing to make for the already small demographic that is adults at their TVs. Hazbin Hotel is an anomaly. It's a web project that got massively popular on its own, whose creator rakes in the cash from her patreon and merchandise sales. It's already got a huge following, but whether that following warrants the show getting a large budget remains to be seen. If the show proper will be anything like the pilot they better be expecting a surge in subscribers cause that's a massive budget for one show to have.
Also, you're overestimating the value of the "Christian market" in America, let alone the more devout Christian and Islamic markets in the rest of the world. First, I doubt most Christian Americans are devout enough to mind a show like Hazbin, considering how cartoony it is, and the ones who are are in the minority, not to mention Hollywood has had their focus shifted to liberals for a decade. Hell, just look at the seasons of Rick and Morty which have liberal-pandering content.
Second, there is far more liberal American media that gets published worldwide that, even when censored in the aforementioned foreign markets, can still turn a profit. And in the rare cases where they do get banned, it's not a huge loss since no one's going to lament not turning profits in Saudi Arabia or Russia. In any case, no one knows for sure how the distribution of Hazbin will play out or how it will be received internationally since, again, it's still an anomaly.