I wouldn't really say that honestly. True her kit is quite good with maintaining a good distance however but her main problem is the sheer lack of offensive capability. Her Q is her only offensive skill. Her sleep is quite limited in range and requires a wall to be able to throw it farther away. It has a very small hit radius and is very easy to spot and avoid, especially in the top lane.
That said, Zoe's ability to trade damage has to be around minion waves and champions like Darius, are able to avoid damage behind minions or by getting in range and pulling you in. Even with a Yorick in lane, if he plays safe and bides his time, he does scale against her. Her only damaging ability being Q, is also an aoe which are not nearly as effective on Yorick's minions, and a well placed mist toss, will send his minions straight to you. Gnar also is able to very easily close distance and completely force Zoe on the defense or even poke her at range and avoid her sleep skill by knowing the tell for its use.
Is Zoe fairly good in the top lane? Sure, as effective as most mages are, but like them, she also has similar weaknesses, mainly being she has a limited response to getting closed in on added by the fact her sleep spell, does have a very narrow radius to hit, otherwise it'll just fly past them at close range.