Mary is being her usual self, and as BridgetD mentioned, is misjudging her company. It's not only that she is treating what were considered "serious subjects" at that time period with such levity (Julia is not poking fun at spirituality), but she states her opinions as if they were certain facts, and would be considered so by everyone else. The dig at clergymen's looks is particularly brutal. Fanny, of course, is doubly angered on Edmund's behalf, and I think that her strong reaction to Mary's comments is perfectly understandable.

Mary's remarks hint at shaky religious and moral foundations in the Crawfords. In Tarn's excellent post below, she suggests that the taller Bertrams represented the Tories, and the shorter Crawfords, the Whigs. The Prince Regent (not Jane Austen's first favorite) was, I believe, a Whig supporter, espoused a lavish lifestyle, and did not have a good relationship with his wife (Admiral Crawford?). I won't go farther into this analogy here as I have not read too much about these issues, but MP may have stronger political undercurrents that what I've been used to think.