"Slop" to me means very low quality food.

Pigs get fed "slop" because they can and will eat anything and don't give a fuck how horrible it might be.

If someone is eating something that gets likened to slop it's likely been so badly cooked it's turned into unidentifiable sludge that is still edible but you'd be hard pressed to know what ingredients were involved from the appearance, texture, taste, and smell.