I thought this most unusual also. I think your theory that a mother, however silly, is still Mum, must be the correct one. My reasoning is because of this line,

"Had she found Jane in any apparent danger, Mrs. Bennet would have been very miserable; but being satisfied on seeing her that her illness was not alarming, she had no wish of her recovering immediately,..."

That seems to say that Mrs. Bennet did have the faculties to properly evaluate Jane's condition. Also in Lizzy's mind, Mrs. Bennet would surely have more experience with illness and I think that Lizzy must have wanted to take no chances with dear Jane.