And popularity has nothing to do with private, subjective quality evaluations of the individual (aka, what someone calls slop might be picasso to another), but with objective, public evaluations of the product via purchases.
For example, an AI picture of a bush is not slop, because we don't expect much from a picture of a bush (not aspirational).
A hand-drawn picture of a knight in armor by an enthusiastic, but not very skilled artist is not slop either - it has tons of details that resolve in an interesting way, and what it lacks in details, it allows the viewers to fill in for themselves.
A 'realistic' knight generated by AI is slop - it contains no imaginative detail, and allows very little room for personal interpretation, and it's not rewarding to view.
Slop doesn't need to be AI - creatively bankrupt overproduced garbage counts as slop in my mind as well.