Congress's relevance has been in question, but I wouldn't point to the fact that it has lower approvals than Bush.  In fact, I believe the approvals change a fair bit once you start asking about each party is Congress, but even that is silly.  (The Democrats aren't Republicans, at least in name, so they'll have higher approvals in Congress.)  I believe Congress typically has lower approvals than the president, just as the Supreme Court typically has higher approvals than the president.  It's more of a difference in how the public has historically perceived the three branches.

Also, the president can control his own message.  It's clean.  Thumbs up or thumbs down.  Congress, obviously by its nature, can't, so it's muddy, and "muddy" is usually "bad" for approvals.

Be nice to America. Or we'll bring democracy to your country.