Average skater for a NHL'er, or average skater for a prospect? If it's the latter, i disagree - he is a good, strong, skater with good edges. Not outstanding, but above average for a prospect.5'11 is undersized. I think him being an above-average skater is up for debate. It's not like he's a Matthew Wood clear awful skater, but there's a debate whether he's a high level skater for his size. He may only be an average skater for his size, which isn't ideal. If you are picking someone in the top 10 and they are 5'11, you'd prefer skating being in the strength category.
If it's the former, then ya, but with most prospects you project improvement in their skill sets. I think he will be at least a slightly above average skater in the NHL, if not a bit better than that. Not elite, probably.
So many of our perceptions hinge upon how smart a player. If he is able to perceive and anticipate play better than most, he will react better, making him appear to be a better skater. If the above attributes are poor, he will always appear to be a half step behind play, and sluggish.
I think Helenius is one of those players who has above average IQ, allowing him to play and appear faster than he is(though he is not slow to begin with). His skating will not hold him back in the NHL, at all.