I agree, I think his play has dipped recently as the team has sagged. Chytil as well. Cuylle has stayed pretty steady.
But when Kakko sagged, so did the line. I don't think that's a coincidence. It's not like they've been carrying him around all season. In Buffalo, I thought they were all really meh and Kakko more than the others, but Kakko also managed to set up two really dangerous chances for Cuylle in the slot that Will just didn't bury (which isn't a knock on Will), and it's clear that despite what anyone here insists, his linemates very much defer to Kakko to be the playmaker on that line, and he's done a good enough job at it this season.
Just for reference he's scoring at about the same rate (per60) 5v5 as Lafreniere last year and his primary assists rate currently leads the team.