Regular Willow looks very unwell in that scene. I think this is showing us Buffy's perception of how Willow would deal with knowing about her future evil self.
Good Willow looks like she's dying, while Dark Willow looks happy as a clam. Foreshadowing, maybe? This is Joss remember... just because it's comedy doesn't mean there's nothing significant there...
I agree. She looks like she is whithering away to me...almost like a corpse. Also, should we be noting which demons were present at the wedding? Is there anything in that? Also, Willow was on the grooms i.e. Warren's side of the church? Is she the ultimate betrayal that we have been warned about?