While I wouldn't go as far as to say that mutilation of the dead was common during the Revolutionary War, it was certainly not a rare occurrence. I think that Warren's case, if it is to be believed, (Benjamin Hichborn may have been exaggerating in his letter to John Adams about the desecration of Warren's corpse, considering he was getting the information second hand to begin with) is particularly vicious, but mutilation such as scalping occurred in other theaters of the war. Given that scalping was a battle practice among some Native American tribes involved in the war, it should come as no surprise that scalp-taking often occurred when they were involved in fighting. During the Cherry Valley Massacre of 1778, for example, Native American warriors fighting alongside Loyalist troops scalped the individuals they killed. During the same expedition, other officers recorded that Patriot troops took scalps as well. In the aftermath of the Battle of King's Mountain, the Patriot forces desecrated the body of British Major Patrick Ferguson, the commander of the Loyalist forces, by stripping it and urinating on it before throwing it in a hastily dug grave. At the Gnadenhutten massacre, Patriot militia slaughtered a number of innocent Lenape natives and proceeded to scalp them and burn their corpses. Similar to the Sullivan expedition, the Crawford expedition to attack British-aligned Native American tribes in Ohio Country involved numerous desecrations of the dead perpetuated by both sides (and torture and burning people alive as well). Patriot militia forces scalped Native American dead and the Native Americans returned the gesture. When the natives eventually captured the Patriot militia leader Crawford, they executed the men that were with him, scalped them, and then tortured him for almost two hours before burning him alive. The American Revolution was a bloody, violent civil conflict with what we would now consider numerous violations of human rights on both sides of the war, so it should not really be shocking that these incidents of desecration occurred as frequently as they did.

If you want just a quick, general summary of how nasty the war got, this brief Slate article is a good summary (but doesn't event touch on the border war stuff): http://www.slate.com/articles/news_and_politics/explainer/2000/07/did_the_brits_burn_churches.html

Edit: Worth noting that desecration of the dead is still pretty commonplace across military conflicts to this day.