I don't consider myself an authority on this, but I take it to be a gentle bit of comedic irony, used to describe aesthetics that are generally the opposite what people associate with "hardcore," the best known earlier use of the suffix. Compare hardcore with, for example, prairiecore, cottagecore, dadcore, nerdcore.

All of these terms describe an aesthetic that references the stereotyped essence of a way of life. But these new uses reference a way of life typically stereotyped to be approximately the opposite of "hardcore."

All of them put a little smile on my face, but prairiecore is my favorite: https://aesthetics.fandom.com/wiki/Prairiecore

Edit: Language is fun. There are eight replies to the parent comment so far, and almost as many interpretations of "core," with different connotations. We're all sitting here understanding the word "cottagecore" perfectly well, but having subtlety--yet importantly--different subjective experiences while doing so.