I believe it's a reference to his pro-cannabis stance. Hemp is a strong durable fiber historically used in ropes (and nets). Hemp and cannabis are basically the same plant: marijuana. Mike's line is significant because he is famous for is surf stoner persona. I think the line is an oblique reference to this. I only got it as an adult.
Another explanation I've heard suggested is that it's simply because Nader is known for consumer safety activism, thus the line was about how durable (ironically) the net is. But I personally don't think this explanation makes as much sense as the first one above, especially since it's said by Michelangelo.
It has nothing to do with cannabis or marijuana. It is a children's movie after all. Michaelangelo probably said it because Nader is a well known enviromentalist, and the fact that the scene involves 4 turtles trapped inside a net.
For a brief moment I thought they wanted to hang him, but I couldn't think of a reason why anyone anywhere would want to do that... Then I thought maybe hemp.