Pigpen doesn't speak much, up to this day I have no idea what his role really is.
We do know for one that he doesn't like to play hide and seek coz the trail of dust and dirt he leaves behind makes him easy to find.
Maybe he is just the representation of that kid who never liked to bathe, we all had a friend like that when we were younger.
From the official peanuts site:
Before grunge was cool, Pig Pen made his debut in the Peanuts comic strip on July 13, 1954 and since then has been the butt of "dirt" gags. He walks around in a cloud of dust, sprinkling dirt on all he comes in contact with. Pig Pen is happily messy. He doesn't try to explain it, hide it, fight it. For him, it's just a fact of life. His slovenly ways paid off in 1993 with a series of television commercials for Regina vacuum cleaners which combined animation with life-action.