I am also surprised to see Juventus on a poor performance against City. Maybe, they didn't take the match seriously because they know that they will qualify to the knockout stage. Juventus played with all seriousness in the round of 16 against Madrid, but they failed to take advantage of their chances in the first half to find the back of the net. Madrid dominated the second half and made proper use of it.
Juventus is yet to get it right with consistency of which has been occurred severally with their performance even last season but then it could be for reasons like you mentioned but I hope they do not loose an important game for this reason because it will cost them , looking at the pattern they have been playing in this tournament, you can say for sure that it is possible that they choose their performance depending on the magnitude of the game and what is considered to be at stake, Juventus hopefully do not crash out soon, maybe they will build more strength and confidence by the end of this tournament.