Ottavino's numbers last year do look bad at first glance, but he did give up 6 of his 12 ER in one outing (in which he logged 0 IP). So his 5.89 ERA for the season was a 2.94 in all his other games.
And of those remaining 6 earned runs, he gave up 3 more in one other game. So in his other 22 games, he only gave up 3 ER total in 17.2 IP (1.52 ERA).
He really did not pitch badly last year but instead just had 2 bad games...