No they're not. they're weapons of airplane destruction. Not even the UN thinks of Stingers as "weapons of mass destruction".
It's amazing how a term can be abused and misused by the general population once it escapes into public use. There are three recognized types of WMD's...nuclear, including both explosives and radiological weapons, biological (germs warfare), and chemical (poison gases and such).
Napalm is not considered a WMD, even though it was used to kill hundreds of thousands of people in WWII.
Stingers? A flying firecracker, nothing more.