You are right Tom.

Finally we see a side of Edward which I like very much.

This teasing joking Edward is very unlike the person one forms an image of, when based on the previous chapters.

What is even more interesting is that he and Elinor have the same attitude towards Marianne's sensibility.

While Marianne sees poetry in the falling autumn leaves - Elinor sees them as dead.
Of course she is also joking here, and pulling Marianne's leg. I'm quite sure that she admires there beauty as much as the next person.

While Marianne sees beauty around, Edward sees;
"among the rest of the objects before me, I see a very dirty lane."

He too is pulling Marianne's leg I think and deliberately pointing out the dirty lanes.

So, yes!! Edward and Elinor are both having fun when they discuss these things with Marianne.