First off, you are partially correct. Zerglings are effective against hellions only if you can get a surround. If you cannot get a surround, blue flame hellions will tear you apart. Roaches are great against hellions, they take little to no damage and are effective counters. With Thors mixed in, roaches are still a ok counter. Unless you have mid-late game tech, broodlords, etc. then it is really the only counter.
My reasoning behind saying roaches are the only effective counter, is that lings will die quickly to almost all mech, however they do absorb damage and can fit well for lower numbers of mech and assisting roaches. Without tanks in the mix, mech can be alot easier to deal with with mass roach, with a few lings mixed in, and infestors with neural parasite can help, but do not underestimate fungle and infested terrans as well. Spawning infested terrans soak up damage, and when hatched, do a good amount of DPS. Fungles can lock down hellions and keep them in place and neural parasite can work alot, but if the opponent is smart he will focus down your infestors.
However, for your question, I believe you are asking what is the "easiest" counter to mech. There isn't one. Roaches can hold off mech, but nothing truely counters all aspects of it. The best way to manage mech is by utilizing lings to do major counter attacks as mech is slow. Also, don't forget about banelings. Banelings can come in and knock out clustered thors and even hellions. Utilize hotkeys, if you aren't already. I use space-bar for my infestors so they are always easy to access.
To sum up
There is no easy counter. SCII is a game of intense micro. The higher up leagues you go, the harder it will get. It requires you to have a high apm and the only way to successfully get there is to practice against it. As platinum you should be able to push with roaches while using infested terrans and fungles. You should also be able to keep a separate hotkey for lings to backstab the second he moves out. And if the opposing army is big enough to have a large number of thors, perhaps the question is that you should be teching to a lategame army faster then you currently are. Broodlords are an effective counter simply because they are ranged and they are air. The only thing that typically targets them are thors in a mech army until he gets vikings. The answer to mech is simply to utilize your enhanced mobility and to micro well as I explained above.
Also, I highly recommend you watch Day Daily #464-Mass Queen vs Mech