An emphatic YES. He is most certainly overrated.
Is he a great QB? Yes. Is he the best QB in the NFL today? Probably.
Is he what made the Patriots offense go? Absolutely not.
Matt Cassell is absolutely horrid. He probably doesn't even belong in the NFL...there's a good reason he backed up Matt Leinart (who also obviously has no NFL future and is a complete bust) at USC. His accuracy beyond 5 yards is abysmal.
Yet he has still managed to put up more than respectable numbers (QB rating is now 87.3) in that offense.
It's not difficult to complete WR screens to Wes Welker. He has done this.
It's not difficult to complete jump balls to Randy Moss. For some reason, though, he has not been able to do this.
If he were able to do the latter, he would be posting MVP-caliber stats. Maybe slightly less impressive numbers than Brady (after all, Brady takes maximum advantage of the talent around him), but what is it really saying to say that Brady is better than Cassell?
QB is an important position only in that it allows the offense to run at or near maximum potential when the QB is solid. Quarterbacks can hold back an offense, but they aren't the driving force in what makes an offense good. You can take the best QB of all-time (I'll say Brett Favre from 1995-1997) and put him in a mediocre system with a mediocre offensive coordinator/play caller with a mediocre supporting cast, and he won't do much better than, well, mediocre.
Even with the best QBs, it's extremely rare that you see them "throw somebody open."