What are "core NPCs" for you? Important characters in the city?
NPC stats can be important to get some guidlines on how the players can fit in the world. If your major NPCs are mostly CR 15+ stats that is quite different frm them being CR5. In the latter, a level 10 adventuring party might practically be able to run the city, because they can solve problems (and become problems) that no one else can deal with. In the former, the party might wonder if they are even needed for anything (and then your line-up of NPCs and plots needs to account for that.)
What is also important along the way is all the "lesser" NPCs that they might encounter. You might not need stats for shop keepers, black smiths and barkeepers (but maybe you do, sometimes?), but obviously criminals, mercenaries, guards, military forces and maybe also members of temple hierarchies, cults and wizard academies.
It will make a big diference if the average watchmen is CR 1 or CR 5 or CR 15, obviously, or whether the Guard Captain is Level 1, 5, 15 (and not always does authority equal ass-kicking, maybe the guard captain is just a bureaucrat that barely remembers the last time he pulled out his baton, but he has an elite guard under his command that is much higher level, or a skilled investigator that is trying to identify all the dangerous players and had his share of experience). Finding the right spread so it feels believable and there is a niche of enemies and allies at all levels isn't so easy. It will also inform to what extent the city stays an adventuring location where all the exciting stuff of a campaign happens, or is there a point where it might be more one of the locations you care about (and maybe go back for rest) when you deal with challenges of the wider world?