This seems supported since his delusions mirror ideas he had read in Kilgore Trout's novels before the crash. Further, he never spoke about his time travels or alien encounters before the plane crash and gave the reason that 'I didn't think the time was ripe.', which seems like his mind trying to maintain internal consistency. Overall, I thought the story was intended to be from the point of view of a delusional Pilgrim post plane crash, excepting of course the parts told in first person. As a result of this interpretation I didn't think much of the discussions between Pilgrim and the Tralfamadorians on themes such as free will and death, and saw the novel overall as being much more cynical and tragic than perhaps it was intended to be.

But it is more than that, of course.  Vonnegut successfully manages to make the human conception of time seem absurd. (He enjoys explaining ordinary things in ways that made them seem alien. Another example would be his portrayal of American culture in Breakfast of Champions.)

I believe Slaughterhouse was an earlier expression of this intuition, which of course Vonnegut was only being half-serious about (he was always half-serious). As he saw it, the Tralformadorian's view of time was beautiful in that they could see beyond the brutal and relentless way time flows to human beings. And Pilgrim, by jumping to and fro as he does, ceases to worry more than he needs to about the problems he faces in life. It would be wise to assume that this was something that Vonnegut wishes he could have done. By his own account, he was a fairly nervous man.

And I wouldn't be so quick to judge Pilgrim as suffering from PTSD. Vonnegut isn't a realist, he's an absurdist. Though the narrator is by far incredibly unreliable, I think the story is more enjoyable if you suspend your disbelief and just take the ride without making superfluous judgments. Is Pilgrim crazy, or is he really "unstuck in time"? It doesn't matter, leave that question alone.

 Towards the end of the book he talks about how the birds react to the tragedy of the Dresden bombings and his continuing refrain "so it goes" is a reflection of that.

By the time the generation that fought WWII was in their 40's, their kids started fighting in Vietnam. You hear about the war fighting, but the political/cultural fight at home was the real reason the war was 'lost'. White suburban kids protested in the streets, and for once they were the ones getting assaulted by the cops. The pro-war faction was beginning to lose the hearts and minds battle significantly.

Into the mix came Slaughterhouse Five. This was a book written not by a member of the generation protesting, but by a WWII soldier, taken prisoner in the Battle of the Bulge. No one could impugn this man's bravery or his experience, he'd been in the worst WWII had to offer. The picture he painted was of the insanity of it all, and not a picture of inevitable triumph, but of defeat and futility in spite of victory and for the generation that fought WWII this was heresy.

So in the continuing battle between pro and anti war factions that continues until this day, Slaughterhouse Five is part of the zeitgeist when critical thinking about war came into the American mainstream.