The Hissing Of Summer Lawns was brutally, brutally savaged by the rock press on its release. Seems like the folkie Joni was their heroine, and the light L.A. Pop queen of Court & Spark was their darling, but once she got all moody and musically intricate, they felt betrayed in a way that must have gone beyond the music, which I think is very good. This backlash carried over into the subsequent albums, and by the end of the 70s, Joni's sales had dropped, & her critical reputation was that of somebody who had gone off into left field w/little or no hope of ever returning.
Myself, I feel that Hejira remains one of the great albums of the 70s, as well as Jaco's most musical playing. That's a damn fine piece of work right there. Don Juan & Mingus each tail off from the other (I found Don Juan erratic but involving, & Mingus to be pretty much unlistenable), but the reviews in the rock press showed no discernment between albums or musical awareness of just how new and interesting it all was whatsoever.
I suspect that some of this was due to Mitchell's seemingly sudden turn to more consicously "arty" ambitions which occurred concurrently with the whole "back to basics" movement of punk & new wave (which most of the rock press was all over like white on rice), & some of it due to one of the oldest pitfalls in the business, namely, an artist who has established an "identity" that the critics feel a "personal" relationship with suddenly disowning that image and going off into something completely different.
The best critics don't fall prey to this, but there's a lot of critics who are really just avid fans living vicariously, in some form or fashion, through the artists they champion (especially, it seems, female artists). I could never prove it, but I suspect that Joni had cultivated a certain "girlfriend" vibe with a lot of male critics (just as she certainly cultivated more than just an image as a, uh... "adventurous" sexual type among her male peers - the Neil Young story is one I haven't heard before, but it would not at all surprise me if it was true, although maybe it's a Canadian thing & I wouldn't understand ), and her sudden turn from "woman/child" into serious female musical innovator probably produced the same reaction from a lot of critics as occurs any time an insecure/inadequate male loses a girlfriend that they were unworthy of & lucky to have in the first place.