He was supersmart before the accident, but in the Ultimate Universe at least, he can 'stretch' his mind to solve any problem. Hence why after the whole Ultimate Doom thing, he has a brain deformity: He's stretched his mind so far that it's stuck like that.
In the main Marvel Universe, his intellect is NOT due to the cosmic radiation... He's just brilliant...
I do, however, think his abilities and status as a super hero, allows him access to technology and knowledge that 'normal' human geniuses can't get to.. He, Hank Pym, and Tony Stark do use what they learn from alien technology. Hank Pym admitted that. So, I think, this can create the impression that he is more intelligent now than he was before...
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