Adams is infamous as being America's prickliest Founding Father. In fact, sending Adams out to be a diplomat is a bit like getting a shark to guard your blood. There are better ideas out there. Still, Adams was useful to the proceedings. He was the guy who secured American fishing rights. (Source)
Aw, yeah, Adams. Thanks for protecting our rights to Maine lobster and Chesapeake crab cakes.
Weirdly, though, Adams hated Franklin, considering him old and feeble. He refused to visit the man, saying Franklin should be visiting him first. That's pretty petty—and not the kind of attitude you're looking for in a diplomat. (Source)
But hey: fishing rights.
This synopsis kind of sells short Adams's other accomplishments. For all his faults, he was a man of principles (that might have been why he was so unbearable in his personal life). He defended the British soldiers involved in the Boston Massacre in court when no one else would—which was a move that actually ended up showing the world that Americans had upright attitudes about the law. Bonus: he was the first vice president and the second president.
But because the Treaty of Paris was such a diplomatic endeavor (and because Adams was a little too crotchety to be an ace diplomat), he played more of a supporting role in the drafting of this particular doc.