short answer: no.
Long answer: at some levels of play, possibly Yes. Overall likely not though. He is a unique champion in that he has an incredibly high skill cap while also being very strong when played at a lower skill level. Most High skill cap champions aren't very useful if you aren't extremely good with them, but zed is strong at all levels of play. That doesn't mean he's overpowered by any means though, it simply means he's a strong choice to play whether you're gold 5 or challenger (I'll ignore bronze and silver for reasons not relevant to this discussion).
I would guess people say he's OP because a good zed can take advantage of the high skill cap to carry a team (which would be harder on a lower skill cap champion). In general, assassins are harder to play but can make a larger impact on the game when played well.
If anything your friends comments are complements, you're so good at zed relative to them that they call him OP! :)