Was he responsible for the entire decade? No, but he was a big part of it. His drafts were disasters. Only 1 of the 7 players he drafted in rounds 1-3 are still on the team and that player, McCargo, is a bust who is iffy to even stay make the roster. Passing over Ngata for Whitner was a harbinger of the poor decision he made hiring Jauron from the Matt Millen managed Detroit Lions. Jauron had a big hand in the pathetic drafts of 2008 and 2009, so Marv's biggest decision, picking a head coach, had horrible consequences long after he left.
Marv Levy wasn't responsible for the entire decade. It is right to respect him for what he did as a coach in getting this team to 4 consecutive Super Bowl appearances, but he was a disaster as a GM.