has fantasy author Terry Pratchett really said more rational quotable things than Einstein, Darwin, Descartes, Dennett, Jaynes, Aristotle and Sagan?
Yes (emphasis mine). As an author, especially an author of humorous novels, Terry Pratchett has much stronger pressures to generate text which humans find aesthetically pleasing than famous scientists, who can count on prestige to carry their words to the world.
I don't know ... there are plenty of references to people in comments in the style of celebrity fawning, just that they are niche celebrities instead of mainstream ones.
Every single one of the names you mention are community celebrities, and yes, we fawn on our chosen ones. However, when I say that we should include more celebrity references, I don't mean we should fawn on celebrities more (neither the community's or mainstream), I mean that we should be more willing to talk about contemporary gossip, give examples about very widely known people, etc. I know it sounds strange to suggest the former, but I think that having topics which we can use when attempting to interact with normal folk on a non-intellectual level can only help us evangelize, break down stereotypes around rationality, and actually interact with normal people, which I'm told can be intrinsically rewarding. The latter just helps reduce pointless inferential gaps created by culture, not knowledge (for similar reasons, we should avoid the accumulation of jargon).
Less Wrong is as a community extremely nerdy. That's true for almost any definition of "nerd" that captures anyone's intuition for the word. However, to a large extent, many aspects of nerdiness are not connected to rationality at all, even though nerdiness may be associated with more rationality in some limited aspects. For example, fantasy literature is probably not in any deep way connected to either intelligent or rational thinking except for historical reasons.
Yet LW is full of references to science fiction, fantasy literature, anime and D&D. In one recent example, a post started with an only marginally connected tidbit from Heinlein. Moreover, substantial subthreads have arisen bashing aspects of other subcultures. For example, see this subthread where multiple users discuss how spectator sports are "banal" and "pointless". I suspect that this attitude may be turning away not only non-nerds but even the somewhat nerdy who enjoy watching sports, and see it has harmless tribalist fun, not very different than friends arguing over whether Star Wars or Star Trek is superior which has about the same degree of actual value here.
There's a related issue which is a serious point about rationality and human cognition: Our hobbies are to a large extent functions of our specific upbringings and surrounding culture. That some people prefer one form of fantastic escapism involving imaginary spaceships isn't at some level very different than the escapism of watching some people throw and catch objects. Looking down on other people because of these sorts of preferences is unhelpful tribalism. It might feel good, and it might be fun, but it isn't helpful.