I thought wearing a cape was normal in this fantasy world, but everyone else dresses the same. He's cosplaying! Like carrying a briefcase and wearing a tophat.
as im understanding it, the boy was pretty isolated during youth being blind and all that jazz. only ones that were there were most likely the maid, mirika (although rarely), and immediate family. it's more than possible that he grew up being read fairytales and the like, as part of his "manners" training by the maid in chapter 2/3. i find it cute LOL