If you are discounting what Mariotta did because he didn't play in the SEC, then you are missing the last 7 years of SEC football which have seen lots of QB's put up REALLY impressive numbers. Go back and look at the top QB number from 2008 (Tebow and Stafford), 2009 (Tebow and Mallett), 2010 (6 Qb's over 2800 yards and G MaC with 70% comp %), 2011 (Tyler Wilson), 2012 (Manziel, AJ, and a total of 10 QB's with 3000+ yards), 2013 (Manziel, Mett, AJ, Murray, oh and Connor Shaw with 24 td's to 1 int), and this year (Sims and Prescott).
While defenses in the SEC are GENERALLY better statistically, QB's can thrive in this league. Mariotta would be just as good, imo, no matter where you put him.