How could LN mitigate NAT transversal problem?
first of all if your lightning node sits behind a NAT it can still connect to other lightning nodes and also open channels with them. once you have channels you don't need a public IP address but your node id is enough. your channel partner and nat will solve the rest. 

The problem is that your node behind a NAT does not have a public IP address. therefor you need to configure port forwarding in your NAT.

edit:
Also you can run your lightning node behind a toronion to have a static address.