I was thinking about this, and the definition I came up with for slop is 'aspirational and highly detailed content that resolves its details in an uninteresting or nonsensical way'.

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.

Old men looking at a child with training wheels and sneering that they’ll never ever be able to ride a bike.