My experience might not be good enough for you, but I had people go mass mass muta against my thor army, with magic box and they simply evaporate. The surface area of the thors gets to big for magic box to be effective, every muta shots at a different thor and they do almost no damage. At the same time you repair your thors, so they are almost unkillable. A pack of mutas is fine for harassment, but at some point they meching player can just k,ill you.
At this point there should be broodlords and infestor support, right? You can't just instantly have mass thors, you have to be on 3 base for that, at which point Zerg will rush to broodlords when he sees it's not a 2 base mech. You're right, but mutas work for the midgame and harass to dely Terran until they can get Broodlords up.
Infestors on the other hand are much more scary and if you have bad positioning np will wreck your army. The same with broods. Against that you need a bunch of ghosts and some lucky emps.
Bad positioning will lose for Terran in any game or any style or against any composition. Once Broodlords are out, Zerg has a good chance to fight back against mech. It's the time before broodlords that really sucks for Zerg against mech.
I suppose you watch any recent mech game, you will see that roach infestor is the way to go.
Please link me to any replay where Zerg won going roach/infestor. The catz vs goody games? Because in the Nestea games, he had great success with ling/muta. From my experience and other games I've seen of it, Ling/Muta always is the best response until you can get those BLs. The only time roach/infestor really works is when Terran doesn't have enough siege tanks or gets caught way off position.