They make her early-game bad; but she catches up with 1 or 2 items.
But you know you said it yourself, she was 0-6 in the same game that J4 was 4-0. Then after she killed him, she was 1-6 and he was 4-1. If she became 0-7 and he continued as 5-0, would you say the game was balanced? Ignoring that Fiora is actually 0-6 and J4 is 4-0 but somehow Fiora needs the nerf . . ..
I think she's mid- to late-game. I do not really think champs should be balanced to be good late game and poor early game, but that's Riot's model.
I don't like getting my butt handed to me early game when I am Fiora. But it is satisfying handing butts later game. Though she's not necessarily going to win every trade or even team fights now.