If you look at DRG/True's games from the recent Redbull archon playoffs, a lot of their wins, especially ZvP, came off doing massive harass damage. Both Mutalisk and Burrow Roach managed to get 30+ probe kills in several games. While you could say that harassing effectively is easier in archon mode, this should be balanced out by the fact that the defender also has the same advantage in being able to stop harass.
While Zerg might not have Terran's huge array of dedicated harass units or Protoss's insane warp prism, by no means is Zerg unable to get lethal harass damage done in the mid game.