To me, what moo was saying is pretty clear: they are saying that most of the monsters do not "think", and the ones that do are usually seen as strange and ends up alone. The only exception being probably the goblins (which may or may not be just humans with a different name, the jury is still out on that).
Generally speaking, monsters are skirting at the edge between sentience and instinct, like intelligent animals or the first ancient hominids. They don't "think", but rather "react": they don't plan, they rarely learn, and when they do it is knowledge obtained by chance rather than actively researched. They don't have "societies" , but rather "gatherings", "colonies" and "herds"; there is almost no mutual assistance, at best some rudimental attempts at cooperation towards a common goal. Some don't even have objects permanence. The ones that have developed farther, they did so due to human influence: the most advanced societies we saw were the ones closest to human territory, and regressed as the protagonist proceeded further into his travel. At the beginning we had the werewolves, which were basically furry humans, with a stable village, winter preparations, a common lenguage, tradings, rituals, social norms, etc, etc... The stable settlements were the first thing to go, soon followed by the single common lenguage. We then lost the ability to properly collaborate and the ability to not eat each other (initially as ritualistic cannibalism, soon devolved into opportunistic cannibalism). During this last leg of the trip we lost any form of common lenguages, the ability to plan for winter (enter in a home and hope to survive), any form of compassion for others, the concept of property (and with that the commerce). The introduction of fish people alone dealt a huge blow to the ability to not kill each other for food, the ability to recognize others as "people" and the ability to be self conscious (most of the time). The only exception seems to be the goblins, but for aspect and behaviors I suspect they are closely related to humans.
We have seen very few individuals that escaped this trend, capable of philosophical thoughts: Moo, the goat (or at least, they were suspected of doing some sort of behavioral research) and now kekuu. Susuki sometimes dabble in some abstract reasoning, but almost always as a reaction to a protagonist's input.
The questions that kekuu asks are quite interesting: meat dies, while rocks last. I want to last, why I have to die? Why I am meat? If I eat rocks, does it becomes meat? The ones that would have followed, would have been even more interesting: if I eat rocks, do I become stone? what can I do to become stone? We die because we are meat, or we become meats after we die? If we stop eating each other after we die, will we become something other than just meat? These are the questions that lead away from cannibalism and towards ritualistic burial.
Kekuu really thinks about a lot of stuff, just has a hard time articulating it. Human beings IRL often have a hard time explaining their choices coherently, or just make up excuses to cover up more complex emotions, I think we can cut Kekuu some slack on that front.
