How important do you make any given high-level PC or NPC?

Levels might not exist in-universe, but they can still serve as a rough barometer of how powerful any given PC or NPC is. This is setting-dependent, though.

As one example, D&D's Eberron is a setting wherein PCs are supposed to be a very big deal. In the main continent's single biggest nation, a 12th-level fighter is described as "renowned as one of the deadliest swordsmen in the kingdom." A different NPC, a 12th-level rogue, "is the deadliest assassin in the service of" a smaller yet still important nation. Thus, PCs who reach 12th level themselves are considered some of the deadliest people in the continent.

In Pathfinder 2e's Agents of Edgewatch #5, the PCs encounter no less than 21 nameless street thugs of 12th level, two gang lieutenants (not gang leaders, but lieutenants) of 17th level, 32 nameless prison guards of 12th level, and a prison warden of 19th level. Mind you, this prison is for mortal humanoid captives, not cosmic horrors.

In Pathfinder 2e's Prey for Death, there is a fortress that protects a small, out-of-the-way frontier town, a fair distance from any major trade routes. Its nameless soldiers are 12th level.

Back to 3.5 Eberron, there is the distant, mythical city of Io'lokar. Here, the mortal humanoids are high-level: very high-level. The city's districts are sorted by how high-level anyone is. For example, the second-lowest tier is inhabited by "Characters of 12th to 15th level; sorcerers and wizards of 8th level or lower; clerics and druids of 10th level or lower; bards, monks, and paladins of 8th level or lower." This climbs higher and higher, until we see multiple epic-level characters in the highest district.

In D&D 3.0's Epic Level Handbook, there is the planar trade city of Union. Rank-and-file grunts are 14th-level fighters, sergeants are 23rd-level fighters, and "backup team members" are 31st-level fighters.