He hasn't been. He's beaten up bad teams in the air in the past, but the playcalling has favoured his and the RB's legs, and Lamar has made enough of what he came into the league already good at to make it work.
I've probably said this before but when he first took over from Flacco there was one thing* about his passing game that gave me a lot of hope, he would scan the field and go through progressions, something Flacco was terrible at. However, his mechanics have been pretty poor. In his MVP season, when he was beating up on the Bucs and Dolphins he had time to throw and it showed, but when the pressure was coming in and he was rushed you'd see him throw the ball over the head of his receiver and into the dirt 3+ feet away from them, he'd trust his arm too much over maintaining a platform.
As I said earlier in this thread, his OL was a mash unit last year and his game suffered for it. This year his OL is still not great, but you can see the work he has done in drilling those mechanics so his release is coming from stronger base. His deep passes are on target and have some zip, and he's stronger hitting receivers outside. He still seems to favour the middle of the field, and the zip on his passes has allowed him to make some throws I to really tight windows, but his overall passing just looks cleaner.
*Edit: two things! He also led the receiver on deep passes, and I'd bet if you go back through Flacco's career you'll find less than 15 instances of him hitting a receiver in stride down the field. For all that arm strength he was rubbish at leading his receiver.
