So IQ stands to Intelligence Quotient. Which if youre not aware is a fancy word for the result when you divide. It gets tossed around a lot and abused. However, its just a measurement of your intelligence compared to the average. As in it is, very simply a percentage of how smart you are at your age and development, compared to everyone else. The average is 100. If its less, youre dumb. If its higher, youre smarter. Because its your score divided by the average score. Ergo, if anyone tells you the average IQ is dropping. Their IQ is pretty low. Because the average is 100 and always will be.
Now if youre like at 70 you are clinically retarded. I think Phil is in the 90s. He is just too dense. And actually, concepts like a "rainy day fund" or understanding crouch jumps are the kind of thing an IQ test looks at. And while he manages to feed himself and dress himself, he does seem a bit short on understanding higher concepts. He might be 80, but thats kind of contingent on whether he understands his constant begging is constant or he genuinely just doesn't realize he does it. Which I think helps explain the audacity of his begging in a lot of ways
Lastly, he is decreasing. Like I said. Its based on age and expected development. Phil probably peaked early, so he could br valedictorian, but he never matured and his last step of abstract thought never seemed to develop. That ability to think ahead. He just has no concept of it.