There was a lot of humor in Mass Effect 2, particularly with Mordin Solus (Gilbert and Sullivan, anyone?) and Joker, but it certainly didn't stop there. Pretty much all of the characters had some amusing one-liners at one point or another (although Grunt's were essentially the stuff of nightmares if you looked at them any other way--particularly his "getting it" spiel. Giant, violent lizard nihilists. Scary!).
I don't really see the point in a return to "light and fun". True, ME2 was a "darker and edgier" sequel, but launching us into a happier universe when the Reapers are preparing to shit intergalactic extinction on all the races of the universe is not a good choice of narrative tone. It's always darkest before the dawn, the saying goes, and freeing the universe from the clutches of a genocidal, manipulative race of machines is a hell of a dawn. I'd expect some pitch black moments in ME3, but I bet I'll be hugely disappointed.