Look at how Devon Setugucci revealed recently about his drinking and drugging habits in the past. Or imagine Theo Fleury the last 5 years of his career and how messed up he was.
Byfuglien and Kessel are top tier athletes even compared to their NHL peers. Body fat is not really an indicator of your ability to be in shape to play hockey.
I don't know who the player in the worst shape in the league is, but it is likely someone dealing with addiction and personal issues at the moment. Not Buff or Kessel. Maybe they are not as "fit" as some players but if Buff can play 26-30 mins a night, he is not in bad shape.