Moving to the NFL means a player will be facing a higher caliber of competition than he faced in college; that's true for every prospect. However, Robert Griffin III will be facing much better defenses than he faced with the Baylor Bears, no matter which NFL team he ends up on.
The Big 12 has some excellent offenses, but those offenses are aided greatly by the lack of good defenses across the board in that conference. Only one Big 12 team ranked in the top 25 in the country in total defense this season, while four Big 12 teams ranked lower than 100 in that category.
There is much less parity in the NFL. There are good players on every team. The Green Bay Packers rank last in total defense, and they have guys like Clay Matthews and Charles Woodson.
Griffin benefited from playing against sub-par defenses in college, a luxury he will not get no matter who he plays for in the NFL. Griffin may not be able to succeed when he's facing a defense that knows how to shut playmakers down.