It's likely because joki generally Plays higher in the line up, gets tougher match ups, and therefore looks worse. If he was playing as a 5-6 it wouldn't be as bad. Clague may be worse but he isn't killing the team right now because he isn't being relied upon to shut down top opposition or keep up with our top end talent.
Jokiharju is playing 5-6 minutes more per night than the other two (18:36 to 14:59 for Clague and 13:53 for Bryson) as well as being at 2:01 TOI on the PK to Clague's average of 2 seconds per game and Bryson's 1:03. He's soaking more important minutes and of late not doing it well.