It’s just a set phrase. I/they/it etc. could not have been [comparative form of adjective]. It means is/were etc. extremely/exceptionally [adjective], in the sense that it would not be possible for it/them to be more so.
When our grandson was born, we could not have been happier.
The hospital staff could not have been more helpful.
It's not subjunctive but past conditional. The past conditional ist used for unreal situations in the past. It's easier to understand in a complete conditional sentence:
If I had studied harder, I could have / would have passed the test.
had + participle - - - - - - could have / would have + participle
A -Don't complain. If they had never painted your office, it could have been still more depressing.
B - No, even if they had painted it in bright colours, it couldn't have been more depressing.
It is a type 3 conditional with no "if" clause. It is an interesting philosophical question whether we can speak of some kind of implied "if" clause.
It could not have been more depressing if it (had) tried.
It could not have been more depressing even if someone had specifically set out to make it as depressing as possible.
It could not possibly have been more depressing.