Not kermit, but how i would define slop, is simply when the generation contains "errors". Even more so when these are errors that a human artist would not commit, no matter how novice. There are many types of these. Here are some notorious ones that one can see all the time in these threads

Scale errors : Things like the head being way too big relative to the body, or the subject looking like a giant relative to the background (such as beds)
Continuity errors : Artstyle of the face doesn't match the rest of the body (realistic 3dish body + ultraflat no-nose anime face), background objects not aligned
Low resolution glitches : mangled details such as irises or finger tips due to no/poor upscaling, for instance
Intent errors : subject not behaving like they should given the context (e.g. loli destroyed by giant cock has absolutely no reaction to it, just an empty face or soulless smile)
Background issues : this is the most difficult one to avoid, escherian rooms, subject's legs melting into the ground, etc.
Typical AI issues : extra limbs, extra fingers, you know the ones, it sure doesn't stop people here from posting

If you have any of this in your gen, your pic is slop. Because they can all be avoided with a bit of work, better prompting, or human drawing.

Then there's also the angle that a gen can be considered slop if it does absolutely nothing new (1girl expressionless portrait torso shot).

As the technology improves, our standards go up, and more and more things qualify as slop. My gens from last year are all absolutely garbage slop now, even though i thought they were well above average at the time. The slop never ends.