Is it conceivable that JA is expounding a school of thought without endorsing it? She slips so readily between the narrator's voice and characters' internal voices, that she could equally well slip into a point of view of conventional taste. When people flock together on some matter of taste, without really thinking for themselves, they become a target for JA's wit. We see it in S+S^* and NA^** and MP^***, so perhaps that's what she's up to here, too? Exposing the 'you can only grieve properly if you're willowy' school of thought to ridicule, by showing it starkly?

^*when Lady Middleton thinks ill of the Miss Dashwoods for being 'satirical', without knowing what it means

^**when a prescriptive taste allows Catherine Morland to dismiss all(!) of Bath as un-picturesque