6 players less than 170 have played in the NHL this year.
3 would be considered NHLers, they other 3 have combined for 12 games (3 by Leipsic).
Marner will likely put on weight, and can get stronger.
There should be little worry about Marner getting to the NHL, the only question is when, and that may be impacted on his physical maturity.
There is no rush, he's a kid.
There are 3 main aspects to determine if a player is ready to play in the NHL.
Is he ready Mentally, Physically and does he have the Talent and Skills to compete.
Nobody questions Marner's talent/skill level as those are apparent and obvious to all that have seen him play. You don't put up 2pp average without skills.
So that leaves the other 2 components and one is also easier to determine if you step on a scale and weigh in at 160ish and you know only 3 of 800 current regular NHLers compete at that level. Marner is in complete control of this as to how much work he puts in off the ice in the gym and how focused he is on good nutrition and eating habits.
The last one is totally intangible in trying to determine if Mitch is emotionally and mentally ready for the NHL stage. Last year during the rookie training camp and exhibition games he look nervous and unsure of himself and so Marner failed 2 of the 3 requirements and was send back to junior.
How much time it will take Marner to get ready physically and mentally is the question that remains unanswered.