There's no fluff support.
The rules, I would think, should work just fine.
Not really.
How I'm defining "mundane" is "your character is no more powerful than an especially clever or strong farmer." That definition is key, because 4e's 1st level heroes are well beyond any common NPCs. Their encounter and daily powers, at least, are things that no basic human guard is really capable of.
There's two ways to make this more mundane. The first is to cut out PC powers so that they are more in line with basic effects. This, I'm fairly sure, will wildly unbalance the game when you go up against a group of kobolds with only basic attacks, and, moreover, gets rid of character options, which are a lot of fun to have.
The second is to give NPC commoners abilities that are similar to PC powers, so that every town guard has the Fighter class template (for instance). This gets more than a little wahoo as now every priest in the temple can heal people back from the brink of death and blast holy light out.
This only addresses "wahoo" powers, and doesn't address things like death and dying and healing surges, either, all of which can become fairly important when it is key that your PC feels like any other member of this fantasy world.
Those are pretty fundamental rules issues in 4e. Dismantling the powers system alone would throw things at least a little off.
There's also the missing mundane systems like craft skills to account for.
That's not to really say it couldn't be done, especially by a clever and observant designer (especially one working outside WotC for a 3pp, with no concern about brand identity). But that is to say that 4e still doesn't have support for it. 3e may have only had two levels of support (though, again, that varied -- in Eberron, you feel pretty mundane for about 4 levels because NPC's do have class levels that are above yours. And after about level 7, you feel more wahoo, because they don't! E6 pretty much has the idea that there are 6 levels of "mundane-ish" in a PC, and to stop regular advancement after that!), but it was
something, and something that 4e distinctly lacks.