I think you're right that the mentions of adding to the rented cottage are mostly to illustrate financial foolishness. That said, I feel there are some additional threads here as well.
For one thing, the cousin who owns the cottage is generously letting them stay there at a reduced rent (possibly extremely reduced? not sure the book ever gets into details about that). So if the Dashwoods were able to add value to the building, it would repay him to some extent. Of course their circumstances (and frugality capabilities) are such that they can't actually do this - but if they talk about doing it, and intend to do it, then maybe it helps them feel less like they're taking charity, and like they're not quite so completely dependent on their web of relations and social-capital connections in order to maintain their way of life.
I feel like talking about these cottage renovations as a realistic future plan is also a way to...maintain some of the same social status. As upper class women, there seems to be a level of expectation that they should not have to worry about watching their pennies (at the same time that there's an acknowledgement that falling into genteel poverty can and does happen, and that it's something you should be practical about). And the older two daughters are of marriageable age, so it's a very sensitive time for them - what happens now will shape the entire rest of their lives. So I think their mother would want to emphasize that they're all still...ladies, down to the bone, and that their temporarily reduced circumstances haven't made a dent in their behaviour or their expectations of the level of quality of life that the world rightfully owes them.