I've never thought about Sonny's intelligence before. As Oli says, an emotional nature doesn't necessarily make one dumb. But...

When they talk about drugs, Tom has a well-reasoned analysis about the benefits to the Family. Sonny just says, "There's a lot of money in that white powder."

Vito surely groomed Sonny for years, and we still get the disastrous outburst in the Sollozzo meeting. And the desire to kill Sollozzo without much thought about any other consequences. And his apparent lack of interest in all the business being done during Connie's wedding.

TB is right that sending Fredo away was a good move, but how do we know that was Sonny and not Tom?

I don't know Sonny's IQ but, if he had brains, he didn't seem too inclined to push them to their limits.