She's always characterised as slightly prudish, I think, though prudish compared to Diana rather than prudish by the general standard of early nineteenth-century womanhood. I found that conversation memorable and startling, I think, because it reveals Jack as rather a selfish lover, whereas in every other way he's most unselfish; though, as I say, it's cannily filtered through Diana and Stephen who are both sexually generous (to one another) and in their own ways very self-centred.
I don't think O'Brian really knew her, either.
Though oddly, she isn't undercharacterised, for all that.