Well, remembering back to my childhood of the grungy cabbage patch dolls, I would guess they do get dirty. My daughter generally doesn't take them outside with her. She has gotten one a tiny bit dirty. It depends upon who made them, they should be able to give cleaning recommendations.
I wouldn't worry as much about it tearing and ripping as dirty. They're generally well-made. Alternately, there was a doll in my childhood that my grandmother re-made it's body once it had gotten so very well-loved, so that is an option, too.
Honestly, I try not to worry too much. I tell her to make sure her dolly has on socks, at least, so if she wants to "walk" she will keep her feet clean. LOL.