I don't think there's enough evidence to say either way. There aren't many examples though of players who struggled/were mediocre their first two years and then became very good. Plenty of examples of guys who were mediocre and never got better who started their first year.
2009: Mark Sanchez, Josh Freeman (Stafford the other 1st round QB)
2010: Tim Tebow (should not have been a 1st round pick; Bradford the other QB but injuries were more the cause of mediocrity)
2011: Jake Locker, Blaine Gabbert, Christian Ponder (Cam Newton the other 1st round QB)
2012: Robert Griffin III, Brandon Weeden (Andrew Luck, Ryan Tannehill)
2013: E.J. Manuel
2014: Blake Bortles, Johnny Manziel, Teddy Bridgewater
2015: Jamies Winston, Marcus Mariotta
2016: Paxton Lynch (Jared Goff, Carson Wentz)
2017: Mitch Trubisky (Patrick Mahomes, Deshaun Watson)
2018: Baker Mayfield, Sam Darnold, Josh Rosen (Josh Allen, Lamar Jackson)
2019: Daniel Jones, Dwayne Haskins (flamed out for other reasons) (Kyler Murray)
Unless I'm misunderstanding you, I don't see many examples of guys who struggled/were mediocre their first two years while they "got some reps" who then went on to become good QBs.