Besides, levels aren't a PC conceit. They're a character feature, and they work equally well for PCs and NPCs. Hence the distinction between "Player Characters" and "Non-Player Characters," which were historically mechanically identical except for who's in the driver's seat. 5E has cautiously introduced some optional rules for making PCs be special snowflakes (inspiration, which a DM can choose to award) but there's nothing unorthodox about having an NPC with class levels.
Having 100 0th or 1st level (Variant Human) Sharpshooters would be a bit much though. I wouldn't do that in-game unless I were trying to model a population based on the Balearic Island peoples during Roman times, of whom it was written, "And their training in the use of slings used to be such, from childhood up, that they would not so much as give bread to their children unless they first hit it with the sling." (I'm betting that's an exaggeration, but in D&D we're allowed to take exaggerated myths at face value.)
if a DM wants to assign a ability that works the same way as a feat or class feature to an NPC/monster, he certainly can. But the assumption that NPCs work the same way as PCs, and come with any PC abilities, isn't the recommended way to design NPCs. Adding abilities is a DM-fiat thing.
