Suddenly they remembered how it was cheaper in the country.
Wimpy's anxiety reminds them of other dissatisfactions, like the fact that the country is cheaper than the city.
It's a safe bet that the characters are in the city, because you don't find many apartments in the country. So they are having a kind of grass-is-greener-on-the-other-side moment.
Wimpy was thoughtfully cutting open a number 2 can of spinach When the door opened and Swee'pea crept in. "How pleasant!"
Not that we were getting bored with plain old Wimpy and the Sea Hag, but thank goodness someone else showed up. It's Swee'pea, the baby from the cartoon, who "crept in" on all fours, as you might expect a baby would.
Wimpy was just in the middle of opening a "number 2 can of spinach." Just as a number 2 pencil is your standard pencil, this sounds like your average, everyday spinach can. He opens the can "thoughtfully," even though it doesn't require a lot of thought to open a can of spinach.
When Swee'pea enters, someone – either the Sea Hag or Wimpy, we think – exclaims, "How pleasant!"
But Swee'pea looked morose. A note was pinned to his bib. "Thunder And tears are unavailing," it read. "Henceforth shall Popeye's apartment Be but remembered space, toxic or salubrious, whole or scratched."
Oh no, the baby is unhappy ("morose"), too?!
Being a baby, Swee'pea can't talk, but he has a note attached to his bib. It reads like yet another "undecoded message." It says that thunder and tears have failed to accomplish their goal, so Popeye's apartment will be "but remembered space" – or a thing of the past – for whoever wrote the note. The apartment is either "toxic or salubrious" – that is, harmful or healthy; and either "whole or scratched" – that is, complete or damaged. It doesn't matter, anyhow – the apartment belongs to the past now.
Who wrote the note? The most likely candidates are Swee'pea, Popeye, or one of Popeye's enemies.
The language of the note is a bit pretentious-sounding. When Ashbery uses language like this, as he often does, it's hard to tell whether he's poking fun at something or whether he just likes the sound of the words. He enjoys throwing different kinds of language into a blender, so to speak.