...but I think I finally figured it out.

The advantages of natural folly in a beautiful girl have been already set forth by the capital pen of a sister author; and to her treatment of the subject I will only add, in justice to men, that though to the larger and more trifling part of the sex, imbecility in females is a great enhancement of their personal charms, there is a portion of them too reasonable and too well informed themselves to desire anything more in woman than ignorance.

At first, I was trying to understand what JA meant here. I was thinking "imbecility" and "ignorance" as the same. But I now see that she is saying that the men who are "too reasonable and too well informed" still desire a woman to be ignorant, but just not an inbecile (plain stupid, idiotic, etc.) Nothing about ever desiring a woman to be well-informed or even brilliant. No sir!

This sentence does not treat women well either way! What do you think JA was thinking when writing this... trying to empathize with the men of the time (smiling knowingly, she is being fair; doing them justice, of course) or satirically slapping men upside the head to make them feel a twinge of remorse for having such thoughts? ;)