It wouldn't send the right message to the world community so it'd be a poor decision, but they'd be within their rights to make it. It'd be understandable too, with how long the massacre in Syria has gone on one has to question what exactly it is the U.N. does to fix things. But, again, it'd send the wrong message to the world community.
The US is not leaving the UN in order to promote any good-hearted agendas; they are leaving to escape conforming to international law since the US' survival is based on enforcing their own form of international law. By leaving the UN, the US isn't showing any signs of changing for any betterment; instead, this is evidence that the US will likely continue to or even increase their scale of international, illegal policing.