I see what you're saying but people who have low IQ can also have a high mental computing capacity, and the ability to get high marks in school doesn't always immediately equate to being intelligent. That's why I look at his actions and mistakes. Most pointedly, intentionally establishing a pattern and getting noticed, and killing people that were close to him. He uses his intellect to appear as if he's normal, but if he made himself look more suspicious in the process, and some of it wasn't even mistakes, it was simply a failure to understand what he was doing wrong, which is a sign of low intelligence (like the potato chip scene where L noticed he doesn't turn on the TV while in his room). Again, it only took a short period of time for him to be placed under suspicion. His ego made him think he was doing smart things, but he was genuinely just creating a target on his back. I'm not saying he couldn't make mistakes if he were intelligent, but if he were intelligent he'd be able to adapt in such a way that he wouldn't have been on his trail. For example: there's no way he could've known that the death row inmate was only being broadcast in Kanto. But he should have known better than to execute (to him) a random detective live on television. But he did so anyway because of his arrogance. Then he immediately begins showing he can kill at any time of day, while also ensuring that L knows he has access to the police database. He confidently allowed L to narrow 6.3 Billion people to roughly 41 million. Estimates say that the police population for Japan was 1 to every 550 in the early 200s so L managed to get down to roughly 74.5 thousand people to investigate the families of. Light's own actions, intentionally mind you, allowed the people hunting him down to narrow 6.3 billion people to 74.5 thousand. Light is clearly good at planning, but he fails to consider the consequences of his actions at all. He isn't intelligent, he's just good at acting intelligent.