All we know is that Mal and Zoe fought one losing battle. We know that Jayne participated in a failed heist. We know Simon is a young medical student, and his sister has escaped as a child from a human enhancement program. Kaylee is a skilled engineer who has never left home. None of that is incompatible with being 1st level. Hoban is a skilled pilot whose been doing it long enough to have a good reputation, but normally we don't think of doing professional skills alone as what levels you up so usually "I've been doing a job a while" is perfectly acceptable backstory. Indeed, aside from being a pilot, Hoban seems to be one of the least leveled up characters on the ship. Those are very typical backstories for first level characters. The backstories aren't what implies higher than first level, but rather the fact that we see them overcome other characters sometimes with ease and panache. It's that ease and panache that is the aesthetic some people really want to have immediately. The basics of the backstories though are well within what you could have for a 1st level character and implied off screen adventures actually important to the plot are relatively rare.
The only character with a backstory that suggests he can't be a 1st level character is Shephard Book, who could well be a mysterious NPC.
And in any event, by the "Gandalf is a 5th level wizard" argument that we should always accept the lowest level that explains what we actually see in the story, no one in the crew is more than 3rd level. Wash and Kaylee might well be henchmen NPCs with Expert class. Shephard and Inara are probably run by the DM. And yes, what happens in the series suggests Mal, River and Jayne are above 1st level or that we are using a system that assumes starting PCs are a couple levels of competence above "normal people", but the set up itself is perfectly valid for 1st level characters with no important changes in backstory.
pquote]Secondly, all of this assumes that "telling a story" is what you're trying to do. I see TTRPGs more as depicting people in an imaginary world.