I'm surprised, I thought they would be more or less equal. Is it because he has no real competition among CG fans while L shares DN fans with Light? Or people just like Lelouch more? Can any character even ever outnumber Lelouch in number of favourites?
I think it's mostly as you said. On MAL, DN has 673000 watchers, and CG 500000. Let's assume the same proportion of viewers are fans of the main characters, you don't really have to make a choice in CG since Suzaku is not presented as a rival/equal of Lelouch as it is with L/Light, therefore the potential votes Light could get are halved.
Well, maybe it's also because Lelouch looks more "human" than Light, and is supposedly still a good person deep down while Light is completely rotten from the start - plus, his end is so much less likable than Lelouch's.
For my part, I find Lelouch to be really uninteresting and not making any sense, while Light is at least steady in his actions, and way more consistent. But I guess not everybody have the same tastes ;)
Personally I like Lelouche more because he redeemed himself.
I always like L throughout the entire series, he was always a strong character with clear direction.
Lelouche however, had a definite period in which I despised everything about him, and everything he did. After episode upon episode of seeing him deteriorate as a human being and become more power hungry and evil, it was the best thing you could hope for to see that he hadn't lost his way throughout all these atrocities he committed.
In his final moments he showed his true colours, and that for me is why he was the better character.