In episode 8 of Nisemonogatari, we briefly see Araragi Karen drawn like this:
This looked a lot like some anime (or possibly a character from an anime or manga) I'd seen within the last ten years or so while in Asia, so I have the feeling this drawing style was supposed to allude to something. (Possibly Dr. Slump? I'm not sure since I don't think I ever saw the anime, and I could easily have mixed it up with something else.)
That said, this also looks "generic" enough that's possible that this is only a shift in drawing style. But then right before this image, there's a drawing of Karen that looked like (to me anyway: I have a feeling I'm wrong about this) an imitation of the titular character of Chibi Maruko-chan, so I'm not sure:
Moreover, I remember seeing something that looked very much like a parody of Peko-chan in an earlier episode, though unfortunately I don't remember which one it was:
Is this (the first image, I mean) supposed to be an allusion to some other anime series, and if so, what is it referring to?