This is actually something wich I thought about doing initially!
But because it’s CR based, the issue is even if I title some job as a “commoner”, commoners stop being commoner’s as soon as their CR gets a little high. They can’t effectively be used as a full-pledged job. If you encounter a commoner with some high CR, its probably a retired adventurer with some job on their background.
Usually people portray commoners as any job with a CR 1/8, you can filter by their role: Martial, Specialist, Spellcaster, depending on their background, CR 1/8 (maybe 1/4 or 1/2 depending on whom?), and let the randomizer do the job. This way you have a very wide range of characters available.
Also, keep in mind a commoner created with CR 1/8, but wich you take out their weapon or armor, should probably be worth CR 0.