If the Protoss is not pre-empting Muta play (ie: opening stargate), he will not have enough phoenixes to actually engage the mutas. This means he has to turtle, which means you are going to have a massive economical advantage (plus, you can add corruptors).
If the Protoss has done anything but that, going heavy muta and not dying to a timing attack (5gate, a gate/robo mix, etc) means Protoss will most likely need to turtle for blink/storm, and there is a great deal of damage you can potentially do and little counter-pressure you can take.
I personally find Mutas much harder to handle than anything else, largely because Protoss anti-air is extremely underwhelming.
It is indeed pretty hard to handle considering how inflexible toss' early-mid game build orders go. Supposing you aren't pre-empting muta play. Then I do suppose massing blink stalkers and cannons while you expo/defend your third would put you at a significant economic advantage despite muta harass. In fact I feel that with proper macro play, after fending off the muta harass, counter-attacking at that point to deny his expo could possibly be a game-ending move to the zerg player.
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