Lewis has shown in the past that his Q performance in particular is highly dependent on his mental and emotional state. I think there is some "wear" that has got to him with age, because you can't avoid biology, but I also really think he just hasn't been on the best state mentally this season. Left Mercedes for Ferrari, that's got to trigger something within you. It's the team that gave you all the records and you're leaving them, but you also have to do this last season with them.
Then your teammate is flying, and you don't feel at your best. For a while there you could see Lewis was losing confidence in himself with some of the comments he is been making. Especially that one after Monza when he said they will probably be better with Kimi next year. Things like that are psychological indicators to me.
I think it's too early to discount Lewis just on the basis of this season. We have to see how will he perform next year at Ferrari, and if the run of disappointing performances continues, then all of this bad-mouthing he has received here, would be better justified.