Of course there is a chance that your losses are almost purely a teammate problem, and that your attitude is great, you don't complain in chat, and you are at the mercy of constant people who are worse than you.
However, even if that is the case, and perhaps ESPECIALLY if that is the case, playing a champion in a midlane role who statistically is most likely, aside from Karma, to lose...that is truly pitting yourself against long odds, I'd say. I did the same sort of thing, playing champions and roles I wanted to play, until I was 700 ELO. Then I figured hey, I want to learn Katarina, and went back up 500 ELO. Maybe you will do that with Zed in the long run, especially if he is buffed, but I'd say that being consistently good at Zed and trying to carry games is going to be much harder than being consistently good with a higher win rate AP mid.
This may be the case, and I don't have good stats on mid-lane-only Zed. However, even that high ELO streamer mentioned he thought that Zed was situational, if I recall correctly. He may work with certain team compositions, but trying to carry a general team that's going to be all over the place with random teamfights and AOE damage may be tough.