Quite a few people have commented about possible societal models for such a community. So I'll extend this by adding some thoughts about specific environmental challenges.

One would assume that the conditions in the lower levels are not very nice, so maybe rampant drug usage could be a thing. Some mongrels might be in pain due to the procedure and abuse pain killers. Others may just want to escape reality. Someone has to supply these. Maybe they're a misguided individual who wants to help their peers, maybe they're a stone cold profiteer who will make for an amazing miniboss. Who knows?

Another important factor is food. Mushrooms grow in dark places and could make up a lot of the diet of the mongrel folk. Or a former botanist could experiment with plants that grow under these conditions. Maybe there's even some animal husbandry going on. Or if you want there to be a darker twist, cannibalism is always an option. Maybe it's even part of their society. Being chosen to be the next meal is a great honor.

Lastly, what do the mongrels do for fun? Do they enjoy pit fights?If so, the players could take the wrong way into the arena room and face a crowd of cheering mongrels and some tough opposing fighters. Are there storytellers who remember some fragments of their human lives? Perhaps even some trash collectors who are willing to sell their strange goods even to outsiders?