just for the sake of being 100% correct : "est-ce qu'on est propre ou sale après avoir joué dans la boue" (not jouer).

You are correct. Except in what context could you be using "on" to ask the world at large, or no one in particular, about their state of cleanliness? I think the only possible meaning is "nous", so "propres" has to agree in number with that implied pronoun.

1km : "est-ce qu'on est propre ou sale après avoir joué dans la boue?"

So, yes, both are correct depending on the context ;)

@hugolemieux - Nice example.

@sitesurf: Thanks, i edited