You are free to not criticize but to me that's an oxymoronic expression.
Clean is the sound of your pickups amplified.
People call that sterile if they don't like it and clean if they do.
Anything the amp adds is no longer clean. It's dirty, unclean, no longer sterile.
So what does clean but not sterile mean? It means just dirty enough that it doesn't sound sterile.
Most often it means a lack of obvious dirt but noticeably raunchy in a good way. When I was hanging on the opposite side of the Atlantic bass forum it was common for those tones to be raved about as clean. There were also a lot more threads on what the best XYZ was for Metal.
I like just a hair on that gets a little bit raunchy and not too loud when I dig in. Lots of amps can do that, as can pedals.