Indeed, I'd like you to try a little experiment. Make mp3's of the tracks from NS and from SP. Put them in a giant playlist together, and set it on randomize. Play it for somebody who's not actually familiar with either of the albums. I will bet you dollars to pennies that a set of unbiased ears will NOT, I repeat NOT be able to tell you which tracks are from the "good" album and which are from the "crap" album. The simple reason for this is that there is not a single legitimate reason why the country songs on this album should be any worse than the ones on Skyline, and as a result the album is just as good as its predecessor, no matter what everybody says.
Personally I don't think that Self Portrait is awful, but it's mediocre and even if you edit it down to the length of Nashville Skyline it probably won't be as good, although to be fair I haven't listened to it nearly as many times and can't remember off the top of my head which are the better songs. But it is stylistically similar. It's weird to put out a 27 minute album, and then put out a 73 minute album that sounds like it but is much worse. But if you threw out half the material, even if it wasn't necessarily all of the weaker half, it would seem like a normal but weaker followup to a successful album, and nothing else.
I don't think that Goats Head Soup is disappointing. I hold conventional views on the greatness of the preceding four albums but really did anyone expect them to keep that up forever? There's alot to enjoy on Goats Head Soup, and it's not like it's an inferior retread of what came before it.