Is she a bad actress? I don’t think I’ve seen her in anything.
Anyone who looks that good is going to have detractors (and I have no idea about her politics or the rest of her life) but I've been impressed with her in her role as Betty Draper. She has had some very intense episodes on the show and she has had to become, with each new season, a less pleasant character.
Having listened to much of the audio commentary on the DVDs of the first four seasons, the producers think the biggest challenge is how to make her look less beautiful when the scene calls her to be mean or menacing. The commentaries have often complemented her ability to get more out of a scene than they expected, for what that's worth.
According to SNL cast members, her appearance was their most painful since Steven Seagal. And Seagal is considered the worst host they ever had. I don’t watch “Mad Men” so I can’t comment on that, but her SNL appearance was HORRIBLE.