My take on these lines is slightly different. I see Jacques playing his usual role, which is the foil to the fool, over-dramatizing his view--and he is such a world-weary man!--that the burdens of life are such that he wishes he could make merry, but, sensitive as he is, determined to see only the tragedy of life, he simply cannot. He is being ironic when he wishes he could be a fool,wanting those who hear him to sympathize with his burden of understanding of the great tragedy of life. The Duke will have none of it--he represents what is good and valuable about living, having set up an alternate life in the magic forest where values are balanced in favor of good will and humor rather than those of the court, where money, antagonism, and lack of brotherly love reign.
In this scene, Jacques is not so much himself "playing the fool" as playing the Duke for a fool. His goal is to emphasize what a complete idiot he finds the Duke to be. Jacques, cynical and melacholy about everything, finds no merit in the Duke's decision to forfeit his previous life in favor of "sweet existence." So here, Jacques uses all of his wiles to make fun of the Duke:
"I met a fool in the forest," says Jacques. He reminds the Duke of everything that he's lost, wealth, women, merriment. Jacques cyncially laments, "Oh that I were a fool!" He never had these things to begin with, much less the choice of (in his opinion) unnecessarily giving them up.