Confidence: 1 (3)
Characteristics: Int +2 , Per +1, Str -3 , Sta 0 , Pre +2, Com +3 , Dex 0 , Qik 0
Virtues: Way of Town, Archieunuch (Tso p38), Puissant Intrigue, Puissant Folken, Blessing : Power Aura of Rightful Autorities, Social Contact ( Spy Network)
Flaws: Ennemies ( Master of spy of Germany), Meddler, Obese, Short of Breath, Secretive, Eunuch
Wouldn't agree with your assessment of Tyrion's personality - he's no more lecherous than anyone else and not defined by his ambition. I'd go with Rebellious (specifically, he can't keep his mouth shut to save his life - remember how he threatened Joffrey?) and Judged Unfairly.
Cersei is actually highly intelligent and her ability with Intrigue is on par with Littlefinger's. The problem is that she has far more psychological problems than you can fit into an Ars Magica character sheet - lechery, paranoia, an inability to realize when sex and intrigue are not the appropriate tools...
I wouldn't put Cersei as Lecherous - any more than any other GoT character that enjoys recreational sex. She didn't seem indiscriminate. She is definitely Proud. Almost everything she does stems from the twin prongs of Proud and Dark Secret. She might also have a Weakness for her children, though Geoffrey definitely strained that towards the end.
However, I think she's only 'intelligent', not remarkably so. +1 at most, if not +0. Most of the dumb things she does would stem from Proud, but she doesn't seem to have any moments of brilliance.
Littlefinger seems to rely more on Folk Ken than Intrigue. All of his successes come from reading people correctly. Most of the characters in GoT are actually pretty poor at Intrigue, doing stupidly selfish things that hamper their ability to ascend to power. They all seem to have amazing capacity for shooting themselves in the foot.