They can be ordinary folk, but I think there is room for them to have been more. Like, they could have been a royal but suffered a fall from grace or loss of position.
I think it is folley to allow players to create a grand backstory like "My rogue is the most impressive thief who ever lived!" because as soon as they roll their first skill check that is obviously untrue. That said the rogue (in character) might be a pompous air head who honestly believes he is the most impressive thief who ever lived and who has to overcome his pride and acknowledge his shortcomings so they can actually grow.
Or, if your character wants to start with wealth and power, I might allow it but within session 0 (or soon after) we are going to talk about how they lost it all and how it has created story hooks to be explored later (owed money, old adversaries/enemies, etc).
I think the most difficult problem is when a player wants to basically write some elaborate backstory that goes into way too much depth, and more often than not is them really trying to shoe horn in some anime/twilight character. That shit I won't allow because it's just too unwieldy to incorporate all that into the narratives that we are creating as a group and threatens making the entire game story revolve around a since main character.