Actually, I think I have read some place that the charmingly picturesque was a reference to a grouping of cows that readers of the time would not have missed. I suppose one could also argue, though, that if it was obviously sarcastic, Darcy would not have so easily allowed himself to continue falling under her sway during the Netherfield stay. He did not realize he was being made fun of?
Or, a different argument, maybe he did realize she was having fun at his expense but he appreciated the slice of wit anyway, and was too big a man to hold her intelligently satiric eye against her. A big enough man to laugh at himself rather than a petty one who would have whined in outrage or sullenly sulked -- the former seems more like Darcy.