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In a broad sense, people assuming the worst about a hero would indicate Hero with Bad Publicity.
Your example about Jameson and Spider-Man is listed on the Hero with Bad Publicity page — it seems more of a personal grudge with Freudian Excuse and All of the Other Reindeer reasons behind it rather than Fridge Logic per se.
Well, the Jonah example was just one of the points - Spiderman's a Hero with Bad Publicity, but the fact that everyone assumes he's just one guy is a piece of Fridge Logic considering there might be anyone under the body-covering costume (and in fact, one early issue has Doctor Doom kidnapping Flash Thompson when he dressed up as Spiderman).
For instance, if a superhero story ended with the twist that there are more than one person using the superhero costume, what trope would that be subverting?
Yup, Costume Copycat is what I was looking for, thanks.
Is there a trope about the Fridge Logic that applies to countless superhero stories - that is, the fact that people in-universe always assume the superhero is a single guy, despite not knowing anything about him - and despite the fact that the superhero costume makes it easy for impersonators - for instance, J. Jonah Jameson always assumes Spiderman (the chameleon or Mysterio) robbing a bank is "proof" he's a criminal, rather than some other powered character buying the same costume.
Edited by Mac_R