I think it's impossible to make a general statement to this since playerpools in each live casino and online differs. for example: NL 2 plays differently on Stars then on 888. In some live Casinos NL200 is maybe very soft while in others its a tough game.
In general u can maybe say the skill level of online is ten times higher then live. So Nl2 online would be comparable to Nl20 live imo.
I dont have personal experience with live poker, but its common sense, that most beginners will to go the cheapest game available, whatever that is. And since
online poker is far more convenient, and volume can be much higher, it makes sense, that any given stake is significantly tougher online. But with that being said there are also reasons, why a 1/2 live game might not play as a 2NL or even 10NL online game. The main one being simply the money at stake.
Very few people care deeply about losing $2 or even $10. And for that reason you will often see, that recreational players in online microstakes games will rather sell their son for medical experiments than let you
bluff them, especially at 2NL. Whereas in a live 1/2 game
bluffing might actually be quite effective against a lot of players, since $200 is
real money to them, and they dont want to lose it. Maybe they only brought 1-2 bullets to the casino, and if you stack them, they will need to end their evening early. And even just for that reason they might prefer to fold rather than dig out some big hero call.