IQ is a statistical measure. They make a lot of people take tests, they set the average score at 100 IQ and they give you your IQ score depending on how better or how worse you did than average. The IQ score is calculated with an exponential function but right now I'm too retarded to give a simple and concise explanation.
Technically speaking you could give an IQ test to a group consisting only of severely retarded people, but because they are all a group of retards, and therefore the average IQ is by definition 100, you could have a dude with down syndrome who can barely read a text, score an IQ of 140 just because he did way better than average. But if you introduce more normies and Rick and Morty fans into the mix then that guy's IQ will become lower.
In other words IQ is about relative intelligence not absolute intelligence.