The higher the MMR, the higher the average IQ is.

Bronze and Silver are special needs tier. Gold is borderline special needs. Platinum is a mixed bag, but mostly special needs. Diamond is also a mixed bag, but Diamond II and higher you'll find that on average, a person's IQ is much higher than in other divisions.

Beyond things like mastering a champion, being mechanically "sound," it requires a high IQ to consider the multiplicity of factors that go into winning a game, to constantly have in the back of your mind... "with this next step, I expect this will happen." "I have a mental map in mind, if I cross this point, they will flash ult engage on me, but I have two escape routes in mind, and an alternate one should both of those fail." Team comps don't decide games either, if played masterfully/intelligently, the terrible team comp can prevail.

No one below high Diamond has such thought processes.