Oh, believe me, I do. I know about the origin of Derpy and how it was pretty much fanon from start to finish.

But that doesn't get the writers off the hook. They decided to take this version of Derpy and canonize her, instead of sticking with the absolutely perfect situation where she was a recurring funny background event.

(And I feel it's worth noting that while the Wacky Idiot Derpy is the most well-known fan characterization, I've seen no shortage of others where she's a mentally able character who's at worst clumsy and forgetful, or mentally disabled but treated with dignity and sensitivity. It's not completely a den of horrors.)

So, yes - it's the fandom's fault, but it's also the show's fault. They didn't have to put that scene in. They did anyway.