L isn't as smart as Light. Light is simply smarter. And this isn't downplaying L.
L is the only person in the world of Death Note that could be considered a peer to Light. In a different timeline they'd be the best of friends and the absolute greatest detectives ever. Rivals, peers, friends, all of that. However, Light stood ahead of L, by whichever margin, but still ahead.
Which is why even the creators decided to give him a higher score when it comes to intelligence.
L had every possible resource at his disposal, police, the FBI, endless money, advanced technology, and manpower, and still failed to catch Light. He had access to every police database, surveillance cameras, an army of foot soldiers, and the capability to build a skyscraper overnight just to use as his headquarters. L basically had near-godlike power and resources to expose Light as Kira, and yet he couldn’t make it happen.
Despite all this, Light continuously outsmarts L. Even Rem, the Shinigami, acknowledges that Light has transcended their kind.
L is undoubtedly intelligent, but Light is on another level. Throughout the story, L constantly sets traps, tries to provoke Light, and observes his every reaction, no matter how small. Yet, Light never falls into these traps. To be able to consistently outsmart someone as clever as L, without showing any sign of anxiety or change in behavior - Light simply has to be the smarter of the two.
When Light is finally caught, Near points out that it was only because he and Mello together managed to surpass L. But even then, they didn’t actually catch Light on their own. Light predicted Near’s every move, even before Near himself realized it. He knew that Near would figure out Mikami was the best person to carry out Kira’s will. As a result, Light set the perfect trap by pretending not to know that Near would swap the Death Note’s pages for fakes. He instructed Mikami to start writing in a fake Death Note, all while playing it cool, pretending he didn’t know what Near was up to.
If Mikami hadn’t gone off-script and killed that woman, which Light explicitly told him not to do, the SPK would have never realized that Light had switched the Death Note pages. This would have led to the deaths of all the SPK members and anyone else who opposed Kira.
In the world of Death Note, there are plenty of brilliant characters, but it’s clear that Light is the smartest of them all.