Currently, Fitz is 23rd in Comp %, tied for 1st (aka worst) in Interceptions, and 1st in TDs. He is 12th in YPA which is actually pretty surprising, and 16th in passer rating which I don't put much stock in. I don't see much of a reason to rank him in the bottom 5. It is true that I'd rather have some of the people below him (ex: Andrew Luck), but that is due to "potential" not current performance.
Looking at the NFL QBs list, I have the following players as obviously better than Fitz now: Ryan, Roethlisberger, Schaub, RG3, Dalton, Brady, P.Manning, Flacco, Rodgers, E. Manning, Rivers, Romo, Newton, Brees.
In the "probably better" category I have: A.Smith, Ponder, Kolb, Stafford.
In the "higher potential" category: Locker, Bradford, R. Wilson, R. Tannehill, Luck. These players are not necessarily better than Fitz today but have a good chance to exceed him in the future. At least one of this group will bust most likely.
That leaves the following at the same level as Fitz: Carson Palmer, Michael Vick.
And below Fitz: Gabbert, Freeman, Cassel, Sanchez, Cutler, B. Weeden (could improve).
So, I can see an argument for "Bottom 10" QB, but not "Bottom 5". I also can see a weak argument for "19th best" which is where he is paid... if you only consider the "probably" and "obviously" better QBs.