Lady Middleton was more agreeable than her mother, only in being more silent. Elinor needed little observation to perceive that her reserve was a mere calmness of manner with which sense had nothing to do. Towards her husband and mother she was the same as to them; and intimacy was therefore neither to be looked for nor desired. She had nothing to say one day that she had not said the day before. Her insipidity was invariable, for even her spirits were always the same;... Chapter 11)

I agree with you that there is nothing wrong in an attachment to your children or that making their care your priority in life is a virtue but I do not see that Lady Middleton’s coldness can be attributed to her absorption in her children—devotion to children does not equal coldness to the rest of the world IMO. Considering her sensibilities, I think it is safe to say that Mrs. Dashwood was devoted to her children but I find it equally safe to say that she was never cold in her manner and this is a difference in character rather than the influence of motherhood. Even busy mothers can have a warm manner and be interrested in other subjects and even converse on them when a free moment is afforded. I think Lady Middleton’s coldness is just part of her natural makeup as is pointed out in Chapter 11—she is so insipid that only her children or some attack on the elegance of her arrangements can motivate her to rouse herself. ;D

…and so little did her presence add to the pleasure of the others, by any share in their conversation, that they were sometimes only reminded of her being amongst them by her solicitude about her troublesome boys. (Chapter 11)

Most grateful did Elinor feel to Lady Middleton for observing at this moment, "that it rained very hard," though she believed the interruption to proceed less from any attention to her, than from her ladyship's great dislike of all such inelegant subjects of raillery as delighted her husband and mother. (Chapter 12)