Bear in mind that 2 of those six picks came in this game, in which Darnold was generally awesome against an elite team: http://www.espn.com/college-football/boxscore?gameId=400935260. He was 21-26 for 316 yards, 4 TDs, and 2 INTs. That's a great game despite the picks.
He was even better in the conference championship game against Stanford: 17-24 for 325, 2 TDs, and 0 INTs. Stanford has a well-coached, well-schemed, and talented defense.