I dont think its related. You can have more good or more evil characteristics and still be dumb or smart.
I think you can be an asshole on the inside but still be good due to your actions. I think there is some relationship, for sure, but I don't know how strong the relationship is.
I think if you are smart, you will adapt to your characteristics and your environment to get what you want. So if you are evil, you will use that to your advantage. If you are good, you will use that instead.
I think the argument is more that being good is in your best interest. A bit like, imagine that society was run by cutthroats. Would you prefer that over a society that was good? Irrespective of your characteristics and tendencies, you would probably want to work together with others towards a better society, because it's in your best interest. Despite the short-term gain from fucking people over and lying. I think recognizing that being moral is in your best interest in the longer term requires some level of intelligence. It's like recognizing meta-level consequences, the consequences that are not immediately obvious on a month or even a year timescale.
For sure, you can't be extremely evil if you want to live in todays society or even most societies in history. But relatively speaking you can act on some of your evilness if you are in a position to do so. And on some occasions, your ability to be evil can be an asset. You just need to know when and what to do, like if you can get away with it.
Long term can be better to be good, but you can be just evil enough, that the fruits of your evilness wont show till you die. And you wont care then.
But i also think you can just not give a fuck about society and be far worse, where they tolerate you just enough. If this makes you achieve the goals you set for yourself, then why should you care that much about your self-interest? It is important to note what your goals are, not everyone care about being comfortable, having power, money and bitches. Some care for only a few of these things, or are satisfied with less than the norm, or care about different things altogether. And perhaps some think the effort is not worth the price.
Does it make a serial killer stupid, if he knew that this would ruin his life and he did it anyway and was prepared for the possible consequences? he decided life is not worth living if he does not indulge in the dark deed, for whatever reason. He will do his thing and be happy. Even if the police shoots him. That was a life he thought was worth living and he achieved it.