Kittle has a rough last four weeks of the schedule: Seattle (week 13), Denver (week 14), Seattle (week 15), Chicago (week 16). Njoku’s schedule is much more favorable (CIN, HOU, CAR, DEN, CIN). In addition, I’m not very confident in the 49ers offense, I would bet their performance against the Raiders was an outlier because the Raiders are terrible. On the Brown’s side, it seems Mayfield and Njoku have a great rapore that will continue to grow.
Schedule matters when the players are closer in skill level / talent / opportunity. This not one of those cases.
Kittle is an elite fantasy TE. Just plug and play.