Stewie always makes me laugh. Although, the writers have made him really HOMO over the years. I read a famly guy script on SCRIBD-DOT-COM called To Beer or Not To Beer. I don't know if they're gonna make an episode out of it but stewie wasn't as gay. I WANT THE EVIL STEWIE BACK!
Character wise, Maybe after the Stewie Kills Lois/ Lois Killed Stewie Season 6 finally, Stewie, seeing that killing Lois and taking over the world wouldn't pan out well for him, he decided to take a break from it. Writing wise, they probably ran out of any real ideas for Stewie and the matricidal/ world domination aspect of his character and /or, seeing that gay Stewie had more potential as an ongoing joke, decided to use that instead. Though I have noticed their starting to bring Stewie as a baby genius back into the mix this season with the Multiverse remote and the cloning thing. Hell even bitch Stewie was smart enough to make a clone.
That's why I don't get people complaining, it's not like he's fully lost the whole evil genius thing. It's just now another facet of Stewie's personality, along with the gay thing.
He ping-pongs between one and the other direction depending on the show. I like him more as the womanizing evil genius, but the makers are obviously using the character to push another envelope.