I think part of the Betty Draper character is that she is supposed to stand as something of an anachronistic contrast to the dashing and modern Don. In the first few seasons she is simply obsessed with her thinness and looks and obedience to her husband, going as far as to depend on him to be told who to vote for. The other women in Don's life are vastly more empowered, interesting, opinionated, etc. Ultimately Betty rejects Don and turns to that dimwitted impotent politico who is much more her intellectual equal.
Or perhaps January Jones just has absolutely no talent. Hard to tell.
I don't think that the two possibilities that you describe are mutually exclusive. I have one minor objection, though: Is Betty really in contrast with Don on the point of timeliness? She is initially just what he and other people in their circles expect the wife of a man like him to be -- apart from the aberrations that betray her discomfort in that role and that get her sent to the head shrinker. Apart from that, I think that you correctly characterize the significance of her character.
As for talent and the lack thereof, contrast January Jones's performance in the role of Betty Draper with Vincent Kartheiser's as Pete Campbell. Pete is an even more unsympathetic character than Betty: he is not just shallow, grasping, and self-absorbed but reckless, inconsiderate, and even cruel at times. Yet, revolting as the character is, Kartheiser makes him anything but tedious. We always want to see what he is going to do next (as much as we may dread it), while with Betty Draper, we are just eager to see the end of whatever is bringing her on screen. At least, that is how it is with me.
Pete Campbell is a shallow human being, but Vincent Kartheiser's performance has depth. Betty Draper is a shallow human being, and January Jones's performance is itself shallow.