I think Mary's complaints and Mrs. Musgrove's parallel complaints about each other's servants are meant to disqualify and discredit both their opinions on the subject. While certainly Mrs. Musgrove is more likeable than Mary, her ignorance during the conversation with Mrs. Croft in chapter 8 shows that she really has a very limited worldview that is not necessarily meant to be laughed at as much as anything.

When Mrs. Croft talks about the difficulty of the winter at Deal when her husband was in the North Seas Mrs. Musgrove compares this to the difficulty of coping without her husband when he is at the excises. Hardly the same degree of danger I would imagine. (And where is Julie W with a post about the excises!? I feel lost without your insight. It's hardly a group read without it!)