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It's usually the other way around - they're called crazy because they can see the fourth wall.
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If the Fourth-Wall Observer gets called crazy specifically because they can see the fourth wall, that's Audience? What Audience?.
Not sure what it would be if the character has signs of madness besides seeing the fourth wall (i.e. the other characters would call them crazy even if they didn't mention the title sequence and commercial breaks).
If the implication is supposed to be that seeing the fourth wall is what drove the character to madness in the first place, it could count as Go Mad from the Revelation or Neurodiversity Is Supernatural.
I didn't write any of that.
There's a subtrope of medium awareness, where only a character identified as being crazy is medium aware.
Like:
In "Big Mouth", Coach Steve is aware of the title sequence, and the songs, and he can see the Shame Wizard.
In Gilbert and Sullivan's "Ruddigore", Mad Margaret can perceive the songs in the musical, though she doesn't know it's actually a stage play:
Mad Margaret: They are all mad— quite mad! Rose Maybud: What makes you think that? Mad Margaret: Hush! They sing choruses in public. That's mad enough, I think.
Do we have this?