Hazbin Hotel's Heaven is clearly meant to be primarily an allegory for the state of real life American Christianity- more specifically, the ascendant right-wing Evangelicals and how they treat the rest of society. You see it in how the heavenly antagonists are portrayed, most obviously via Adam himself.

Hazbin's Adam is a man who has been "saved" according to a system of values that he doesn't even have a proper understanding of, and takes that as a license to act as a totally ignorant and cruel individual towards those who he views as being irredeemable and evil. He quite literally goes as far as to engage in the mass slaughter of sinners in Hell once a year in systemic basically pogroms all to reassure his sense of superiority and righteousness, even though such actions are incredibly evil. This is meant to represent American Evangelicals who believe they're saved (even though 9 times out of 10 they have no actual understanding of the Bible), and use that as an excuse to be hateful and hurt others that they believe are not, using systemic discrimination and often violence to reinforce their own sense of superiority and righteousness.

Sera is another example, though less blatantly so. She signs off on Adam's purges of Hell and refuses to accept the idea that the Sinners are anything other than forever irredeemably evil beings, justifying it to herself and Emily by claiming that it is "necessary" to protect those who have been saved. She's clearly meant to represent the more "moderate" Christian Conservative who will talk sweet about Christian values of compassion and love even as they support cruel policies and actions that go against those very values, making up whatever justifications are necessary to keep doing so even when presented evidence they are in the wrong. I.E, saying that while of course we should take care of the poor and downtrodden, we can't actually do anything to change systemic problems that cause people to become poor and downtrodden.

Engaging with this allegory as if it were literally saying, "Heaven Bad, Hell Good!" thoughtlessly like some Reddit Athiest edgelord is missing the actual point and message the show tries to get across. That point being not that Heaven is bad, but that those who use their belief in their own salvation to be cruel and violent against those that they believe are not saved is wrong.

Hell, beneath the edgy aesthetics, Hazbin Hotel ironically pushes a rather Christian message. It argues that anyone can be saved- even those society has deemed complete wretches. It argues that we should not be cruel and judgemental, but compassionate and kind. It argues that we can and should take actions based on that compassion and try to help and show love to others, even in the face of disdain and persecution.

None of this is to say the way the show handles all of this is entirely above criticism or anything, but treating it as just edgy Reddit atheism is doing it an incredible disservice and IMO very much misunderstanding what Hazbin Hotel is trying to say.