my original theme in mind when writing was, in a few words, "being negatively changed because of the inescapable environment you've found yourself trapped in."
as you might know, a pickle jar consists of a glass jar, a solid lid, vinegar and obviously a pickle.
the "spritz of fizz" and "mindless clockwork crowds of air" describe the vinegar, which is a metaphor for the negative environment itself :) it's the thing which distorts whatever is exposed to it for long enough :3
the "glassy casing" is of course the jar itself, and it's maybe not so clear, but to me it represents both the entrapment and the exposure of something so ugly brewing within. the "holeless cap" actually does represent a cap to reinforce the inescapable nature of the situation, and i used "twist" as a double meaning for the lid being fastened tightly AND it aiding in the distortion and the "twisted" nature of the new pickle.
obviously we have the pickle itself but the second stanza exists to show that this used to be a bright, vibrant cucumber and has now been reduced to a "fickle" and "grey" version of itself, a "mossy corpse" as i say.
"Glass on rot,
Skin on flesh." is to highlight the fact that this isn't just describing a pickle jar, but that it is a metaphor for trauma. the comparison here is intended to liken the glass jar to skin (fragile and easy to see through) and the cucumber/pickle to flesh ("flesh" referring to your inner self.) in other words, people are changed within in the same way as a pickle when they brew in the same traumatic situation for a prolonged amount if time, and it is not hard to see if you just peer past their glass front.
i hope this made sense 😭