No... It only diminishes it in your narrow scope. Just about everyone else in the world looked at Stafford's numbers and said that he's a definite top tier.
I would love to see you cite any article from a major sports website that rates Stafford above Brady, Brees or Rodgers. He's clearly below all three of them, which is why he's not in the first tier.
If you think that Stafford's 5000 yards is not diminished by Brees and Brady both throwing for 5200+ yards (and breaking a 25-year-old record), coming off of a lockout-shortened offseason no less, I don't know what to tell you. I guess you must also view Sosa's three seasons of 60+ HRs as one of the greatest accomplishments in the history of sport.
I will make the following bold claims right now:
1) There will never be another instance of three QBs throwing for over 5000 yards in the same season
2) Stafford will never throw for 5000+ yards again
Which of those do you disagree with?
5th in QB rating, 3rd in passing yards, 3rd in TDs?
...for one year. Awesome.
In two of the three biggest stat categories he only had two guys ahead of him and he's NOT top tier?
In 2010, Philip Rivers was 1st in yards, 2nd in rating, 5th in TDs. Guess he's also in the top tier.
In 2009, Matt Schaub was 1st in yards, 5th in TDs, 7th in rating. Welcome to the club!*
In 2007, Tony Romo was 2nd in TDs, 3rd in yards, 5th in rating. Top tier getting crowded!
One good year does not brand you as ultra-elite. While Stafford had a very good 2011 and was able to play all 16 games for the first time in his 3-year career, his passer rating over the previous 13 games he had played (because that's all he had played) was a Brady Quinn-esque 67.1.
I don't know where you were in 2008, but when Drew Brees had 5000+ yards and 34 TDs he won the AP offensive player of the year AHEAD of those guys you mentioned.
Gee, I guess Peyton Manning (a QB) must have been pretty sad to take home his NFL MVP instead of the Offensive Player of the Year. You do realize that the Offensive Player of the Year award is basically the consolation prize, right?
In 2008, Brees was not yet a top-tier QB. He elevated himself with his
consistency of play since then. You cannot say the same for Stafford.
Regarding Schaub, he may not be top tier anymore, I will grant you, but before he got hurt last year, he was on his way to his third straight very impressive season. I will acquiesce on him. I remembered his play and numbers being better than they were.
*And this is my point. There needs to be a body of evidence before you declare a QB to be among the ultra-elite.
One very good year puts you solidly in the 2nd tier (see Rivers, Schaub, Romo). To get into the first tier, you need more than that.