I think it's pretty obvious that the term "slop" has food origins. When you think of "slop", you think of oily, greasy fast food, or disgusting amounts of sugar, syrup, icing, etc. The food allegory strikes again. When someone says something is "slop", they obviously mean mass-produced content that regular people willingly consume at their detriment, because it appeals to our most primitive desires. Something lacking of substance, non-challenging material, "Roller coaster" content.