I think they are teasing Marianne about her ideas of pictuesque beauty.

Back when they first met Willoughby, Elinor teased Marianne, "You will soon have exhausted each favourite topic. Another meeting will suffice to explain his sentiments on picturesque beauty, and second marriages, and then you can have nothing farther to ask -- -"

in 1782 William Gilpin wrote Observations on the River Wye, and Several Parts of South Wales, etc. Relative Chiefly to Picturesque Beauty; made in the Summer of the Year 1770, a and in 1794, published an essay 'On Picturesque Beauty'. His writings are intended to instruct people (travelers? artists?) how to view according to the aesthetic ideal called 'the picturesque.' His theories were very important to the Romantics and those who valued sensibility as Marianne did.

For example:

roughness forms the most essential point of difference between the beautiful, and the picturesque; as it seems to be that particular quality, which makes objects chiefly pleasing in painting. I use the general term roughness; but properly speaking roughness relates only to the surface of bodies: when we speak of their delineation, we use the word ruggedness. Both ideas however equally enter into the picturesque; and both are observable in the smaller, as well as in the larger parts of nature-in the outline, and bark of a tree, as in the rude summit, and craggy sides of a mountain.

If you glance through the first book I mentioned (here), it's full of words like 'grandeur' and 'plantations' and 'prospect' and descriptions of hills and walks.

This conversation is Edward teasing Marianne about her views on this theory:

"Now, Edward," said she, calling his attention to the prospect, "here is Barton valley. Look up it, and be tranquil if you can. Look at those hills! Did you ever see their equals? To the left is Barton park, amongst those woods and plantations. You may see one end of the house. And there, beneath that farthest hill, which rises with such grandeur, is our cottage."

"It is a beautiful country," he replied; "but these bottoms must be dirty in winter."

"How can you think of dirt, with such objects before you?"

"Because," replied he, smiling, "among the rest of the objects before me, I see a very dirty lane."

This is Marianne following her internal script again, and Edward giving her some good-natured teasing about it. We already know that back at Norland, Marianne had thoroughly grilled him to find out whether he had any taste in, or knowledge about drawing. I think this is a continuation of that conversation, with some brotherly teasing on Edward's part.

There will be even more of this to discuss in next week's chapters.

Incidentally, the romantic landscape painter J.M.M.Turner's work would be an example of the picturesque ideals of beauty. I've linked below a painting of his called Tintern Abbey. In S&S 3, when Marianne are sitting near the ocean, he is reciting a William Wordsworth poem to her called Tintern Abbey (officially: " Lines composed a few miles above Tintern Abbey on revisiting the banks of the Wye Valley during a tour, July 13, 1798) which is all part of this same sensibility and devotion to the picturesque ideal--

The lines quoted in S&S3:

And I have felt
A presence that disturbs me with the joy
Of elevated thoughts; a sense sublime
Of something far more deeply interfused,
Whose dwelling is the light of setting suns,
And the round ocean and the living air,