Doubt your group's civil gaming abilities... I'm sure this is one of those "one side of the story" things that doesn't make them seem quite as bad, but anyone who complains about the way someone is role-playing their character (unless it's in a "you're being disruptive" sense, in which case the DM should have taken a much different approach than -- laff -- threatening to dock your Charisma score.)
There's a difference between role-playing and acting -- if you're role-playing, it shouldn't matter whether or not your group is "convinced" by your portrayal of anything. So long as you're treating the game with the appropriate seriousness (not too little, not too much, and what's appropriate varies from game to game and group to group), then I don't see why your group should be on your case.
Here's my gut feeling about this situation: I've heard the "good looking but a rowf to deal with it" explanation for high-Cha characters plenty of times before, but never applied to male characters. The term used itself is very gender-specific... It's a stereotype that we pretty much only see applied to women, and one that evidences the amount of sexism we have today. A charismatic man is one who is persuasive and likeable; unfortunately, to be both persuasive and likeable is still next to impossible for a woman in America (or much of the Western world)... Persuasiveness, and the holding of opinions it implies, is still considered and unladylike and unpleasant (hence, unlikeable) trait.
I should also mention that while I've only heard that phrase, "good looking but..." said about women, I've heard it from both male and female players. As I said, it's a cultural thing, and both men and women grow up in the same culture. I've been guilty of it myself, from time to time (an NPC in the PbP I'm running is female, has strong opinions, and is generally cold and rude -- and while I try to paint her as those things, which are traits, sometimes I catch myself using stereotypes intead, and she instead of having opinions and being cold, she becomes opinionated, and a rowf.)
So, I think you ought to sit back and try to figure out what's going on -- are your character's opinions too strong, or is she simply rude? If she's actually rude, then you probably are taking in the wrong direction for a high-Cha character (although, again, your DM threatening to take points away is silly), and you may simply want to change the direction. If it is actually the case that your fellow players are just being a bit naive, then your situation's a little tougher. You can try to talk to your group frankly about it, if you think they'll actually listen... If you think they'll roll their eyes, mutter something about "politically correct mumbo jumbo" (I hate the term politically correct, by the way, because it implies that this sort of correctness -- not being racist, sexist, whatever -- is strictly political, and not necessarily correct in the rest of life), then, well, your situation is tougher. You can give in and play her more "nicely," you can make a stand that will probably get people fed up with you (whether or not you're right), or you can just go on playing the character the way you want to play her, and just ignore anything snide the other players have to say about it.
It's a judgment call, but the last thing you need to do is try to play the character the way the other players think you should (unless for some, weird, weird reason, satisfying the expectations of others is what makes RPGs fun for you). Just enjoy the game played the way you want to play it, and if you find this is hurting the fun for the other players, talk with them about it and try to find a compromise -- but don't just sit down and take petty stuff like "I'm taking your Charisma away because you're not role-playing my Mother." Really.
(Edit: I wrote this before reading any of the responses, and while some of the intervening posts yield some information which makes this all sound VERY different -- especially that last comment, heh -- I think I'll leave it as is, because a lot of points are still valid, and the rest is hilarious, in the I-put-my-foot-in-my-mouth sense.)
