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Just Meta Fiction as it is?
Wait, are you saying they wear the costumes to avoid scaring civilians In-Universe, or to avoid scaring the Real Life audience?
Meta Four, I meant they do it to avoid scaring civilians In-Universe - which means the costumes are silly, but they have the excuse of wearing them because superhero comics are popular In-Universe.
But, yeah, Scorpion 451's answer of Justified Trope fits best.
If a Super Hero story features a character who wears a costume and has a secret identity because he wants to appeal to comic book fans, despite personally thinking it's silly, what trope are we talking?
I'm thinking in particular about stories where real superpowers are Played for Horror and the characters talk about how "they made us wear the silly clothes so we wouldn't scare people".