Muta harass isn't that good to be honest. July tried doing it today and it seems pretty fail against thor + marine + tank builds. Although he played poorly as well.
I think hydra roach is the strongest zerg play at the moment.
Uh... if you saw your opponent making thor/marine, that should be a cue to potentially halt muta production or scale it back.
Yeah, mutas aren't going to win in a stand-up fight against thors & marines. That ain't news.
But you harassment options still exist against that enemy. Thors+tanks = slow. You fly mutas into his mineral line, get a couple of shots off and scoot away before teh reinforcements arrive. Then hit the expansion. Then bounce back and try to take down a supply depot or building add-on at the edge of the main. Then bounce somewhere else.
Oh what's taht? he's going for a 3rd? swing teh mutas over there and sack it before his army can do anything.
Esp on certain maps, having a group of mutas roaming around to pick off SCVs here and there, pick off depots, add-ons, & maintain map control can be super valuable.
Meanwhile, build up your roach/ling army. When teh confrontation comes, sic the mutas on the tanks and you'll mop up the rest. With the speed, roach/ling is fast too; if you get him distracted chasing your mutas, run the main army into his natural and plow it. Basically you have a fast army, he has a really slow one. Focus your all of your tactics on that. (And expand expand expand)
Or... play 2v2, where the opponents can defend against your mutas all they want. Meanwhile I'm sitting here hard-countering their defense units. XD This combo works awesome for me & my friend.