Maybe. It's a bit unfair to judge a last-minute QB addition to a team with no continuity and a -30.5 SRS (next lowest is 'skins with -12.6 SRS--last year's 3-13 Cards were a -11.5 SRS).
A bad team can make any QB look awful. Steve Young was a bust in Tampa.
Of course, maybe Rosen just sucks. You watched him and I didn't, and I trust you...