He was one of the more skilled players, but I think a bigger part of it was he developed strength early and he could overpower in Finland.
I think the real lesson here is he simply didn't have massive upside to begin with. He was an early developer who everyone labeled with the 'he's this strong now, imagine at 23 when he gets his man strength!' label... when he already had his man strength. In the NHL that advantage is completely negated where 90+% of players are strong and most at as strong, if not stronger than him. Combine that with not a supreme skill set to counter with, you end up with a pretty average hockey player. He's a legit NHLer, but I don't think he's a good one.
Personally my biggest disappointment with Kakko is his shot. It was a weapon in Finland. What I didn't recognize and really should have is that while it is powerful... it isn't accurate, and more than that, it has a weird hitch to it. Which makes so goalies have a clear tell when he's going to shoot and gives them a read. At the NHL level, most goalies can take advantage of that read where lower levels they can't. Unless that can be ironed out, I don't think his goal scoring will take a dramatic jump. He's not a smart enough player or have good enough hands/passing skill to really be a much better playmaker.
I don't think he's as bad as this year showed, but I also don't think he's much, if any better than a 20-25 guy. Maybe that moves to 25-30, but seems like a stretch at this point.