I only generate stats for a commoner if said stats for some reason become relevant in the moment e.g. it took damage and needs to make a Con-based check to stay upright or I need to know how strong it is for carrying-stuff purposes.
But yes, for Humans* I completely take the 3d6 bell curve and the 3-18 range as being representative of the population; on the assumption that if it wasn't we wouldn't use it (or a variant) to generate characters and instead would have and use something else that represented the population.
* - other species have their own ranges and bell-curves for each stat; the idea of those ranges and curves being representative of that species' population remains true, however.