Well without a translated bio for the most part we have only SGYY to go by. Personally I'd have to say Sima Yi was most likely very intelligent; we all know LGZ exaggerated most every person during this time period ( whether it be their talent, their ambition, or their humanity ). Though it always seems as he tends to exaggerate key elements of what was known of them: Example:
Zhuge Liang was an accomplished strategist who pretty much ran the kingdom of Shu while leading campaigns clearly a man of great intelligence; LGZ took that and with his bias towards Shu exaggerated Kongming's abilities to the point he gave him magic powers
Lu Bu was regarded the greatest warrior of that time period, a man that no one person could fight and hope to win; in SGYY he fights multiple people at the same time and they can't stop him, the three brothers were said to have surrounded him on all sides and fought him all at once and they couldn't bring him down...sorry but I think if you got somebody with a sword right behind you while you're fighting two other guys with swords right in front of you, all on horseback, somebody is going to strike a clean hit.
Perhaps it's just me but that's how I see it working, thus I'd have to conclude Sima Yi was fairly intelligent given the display of brilliance LGZ gave him in SGYY despite the fact he was a part of " The dreaded evil Wei dynasty ".