In fact both diaboli and klyros are natural born atheists/agnostics: it's in the settings. Some may have other beliefs but most should be in that line (though most diaboli I know of are Black Flame because of the enhanced Dark Way and affinity to the Black Crystal).
The huge ammount of gods (not just Laanx and Talad, but also all the ethnic gods that came with the different races, plus the Black Flame) should make a lot of literate people to have some sort of detachement from religion (skepticism) or follow religions based either in their parents' beliefs or in persuasion. Not being many prophets around (Phostle aside), the second seems less likely.
Personally I think that the conventional citizens should be:
1. Talad followers: this should be the main trend, as Talad created the city and all that. Talad is like the "good god", so he should be the ost popular. The lack of a Talad Temple (so far) kind of distorts this... as there's no visible monuments to Talad in all Yilakum, in spite of his mythical importance. Krans should be almost always "Taladists".
2. Laanx followers: this should be less strong but still a powerful trend, specially among Lemurs (there are few lemurs around though).
3. Ethnic religions: the several "foreign" races brought with them their own gods. We know little of them from the settings, but many dwarves, elven and humans should be mostly interested in the gods of their ancestors. An important mysterious ethnic god is that of the Xacha (again few Xacha around, but it seems they have strong connections with the government).
4. Black Flame: should be minoritary among Lemurs and rare among others, yet many choose it because they are attracted to chaos, evil and/or the mysteries of darkness (and also because of the extra points re. DW).
5. Agnostic/atheists/skeptics: this should be the main trend among diaboli and (I suspect) klyros. They definitively should not be Laanxists in most cases (as per the settings).
Doubts:
6. Klyros: were they originally atheists/unreligious like the diaboli or rather they had their own ethnic god(s)
7. Ynnwm and other mixed race people (many race chars are supposed to be mixed anyhow): they should be divided in several loyalties, including a strong trend towards skepticism ("atheism"). Ynnwm could (mostly) have been raised in aheism (diaboli parent) and either an elven ethnic religion or one of the mainstream ones (Talad, Laanx) (elfic parent). Other mixed chars could have their own mixed raisings and hence open choice re. beliefs.
8. Polytheism: such a large choice of gods should lead to a tolerant polytheist society, in which some worship/venerate one god, some do the same with several and others are just detached from religion. The lack of temples (other than Laanx') seems to hinder this polytheistic activity, really (and in general to hinder religion in general). There's very little in-game mechanics associated to religion, anyhow, and little understanding of what it may mean.