He did this in his last season at RB too where he had an noncompetitive car, went into a bit of a sulk and got soundly beaten by Ricciardo.
I believe it's a bit more complicated than sulking. He just needs to get the car right for him, he's not an Alonso who will drive anything relatively fast, he's a driver who will have the car tweaked to his liking and therefore leave his teammate further behind him as the season and the car progresses. That works for the team, if he's about 0.3 in front of his teammate, the car usually is competitive. If he's matched or beaten by his teammate 5 races in row, both cars aren't competitive.
Last season Vettel was best of the rest, and Vettel was well ahead of Raikkonen. This season, especially since the summer, Ferrari is much less competive and Raikkonen matches him, so it looks like he didn't succeed in getting the car to his liking. Most of his terrible luck this year was when the car was still best of the rest. So it looks like Arrivabene is not willing to develop the car specifically to his liking, and Vettel doesn't manage to be quick with the car like it is, not even quicker than Raikkonen. It's probably Vettel out or Arrivabene out, and if I were Ferrari, I'd make sure the team works to get the spoiled kid the car exactly as he wants it, and sack the team manager who got a lot more wrong this season. Vettel is not the greatest allround driver, but we all have seen what he can do with a car he likes.
