But Lucy is surely too stupid to realise this innate superiority of Elinor's. That's one of the limitations of small-minded people like her: they wouldn't recognise excellence if it were wearing a clown's outfit and dancing a gig in the street.

There appears to be enough evidence in the text that she only too clearly recognises Elinor as a rival in money (oops, I mean "love"), not the least being her "side glance at her companion to observe [the] effect on her" of her opening of that subject (Ch.22). Besides, why this intimate revelation to Elinor - or, in fact, to anyone at all? She's already revealed it to her sister, and now lives in perpetual fear that she's about to blab the secret to the whole world. Every new disclosure would surely only increase the risk of its public exposure. And don't forget the highly intriguing speculations on this board about her jacking up this whole relation-of-the-Middletons thing and the "chance" meeting in Exeter so that she could get at Elinor.

No, she has a reason for running that risk, small as it is, in this case.