"No better than she ought to be"

[WotNot]

@elfkin477

I’ve read this expression scores of times, but it’s not something anyone I’ve ever met has actually said. Clearly it’s a cutting remark, but I’m not sure I understand it fully. Where does the “ought” come into it when it seems to suggest she’s not a better person than you’d expect? Does it mean her family is low-class and you can’t expect better of those people? Or is it more to suggest that as one can predict, she in particular hasn’t made anything of herself (or shouldn’t even try to)? Or… what?

My interpretation is that the expression means “She’s no better than she needs to be [to remain within the prevailing standards of public decency and virtue]” – i.e. while they have no evidence, as such, they strongly suspect her of enjoying herself in private – or at the very least thinking about it.