Ono Here She Comes
There was once this little band called The Beatles. Ever heard of 'em? They had a handful of admiring fans. Everything was going smoothly until John Lennon's main squeeze, Yoko Ono, started coming between him and the other members of the band.
Gee—ya think Spinal Tap was making any sort of reference to that?
Basically, Jeanine = Yoko. Because this film's intent was to skewer both documentaries and rock bands alike, it's pretty clear that the similarity between this fictional character and her real-life counterpart was hardly coincidental. The parallel was drawn intentionally—again, to make this feel as much as possible like a true story, as well as to poke fun at rock stars and all their silliness.
And Jeanine certainly personifies "silliness." She can't pronounce "Dolby." From her outfits, to her astrology, to her drawings of animal masks, it's obvious that she's not the brilliant mind that David seems to think she is. Sample suggestion from Jeanine:
JEANINE: Uh, what I've done is to arrange a whole load of charts.
DAVID: Wait till you see this, wait till you see this, this is so great.
JEANINE: The band's sign is Virgo, and we see it's Saturn in the third house, all right, and it is a bit rocky. But, because Virgo is one of the most highly intelligent signs of the Zodiac, we're gonna pull through this, with great bond.
Under this guiding philosophy, the band gets booked at an Air Force base and a puppet show (not even top billing). Nigel quits in disgust.
Once Nigel returns, it's not clear what will happen to Jeanine. However, in a 1992 real fake interview with Q magazine, the band reveals that "she makes and sells Irish sweaters in her shop Potato Republic […], sells leprechaun key rings and ceremonial candles in The Crow's Wing, runs a new age boutique called Krystals 'n Kandles (alternately The Drippery), and has started a business that works out the star sign of your computer" (source).
That sounds about right.