The term was coined by Nathan Rabin in reference to films, and that's where you meet the MPDG most often, but being perverse of course I'm interested in her appearance elsewhere. She's definitely popped up in YA fiction, for example Jerry Spinelli's Stargirl, but I'm wondering more about her prehistory: can she really have sprung fully-armed in all her life-affirming zaniness from the head of screwball comedy? And if she did, why then? Was there some earlier figure whose function(s) MPDG at least partially replaced? Looking for precursors, the closest I've got to a pre-movies MPDG is Eppie from Silas Marner, who fulfils some of the essential roles - e.g. giving purpose back to a male protagonist who is stuck in a rut, and doing so not through romantic love but primarily through youth, vivacity and freshness. On the other hand, I don't remember Eppie - despite her strikingly MPDG name - being particularly eccentric or ditzy (though it's been a while).
Alternatively, if the MPDG really is new, is she made possible by some particular cultural moment? Is she a secular figure, or a post-Freudian one, for example? Or is her birth just one of those things that don't require explanation? I've no answers, but I'm pondering.