Not knowing any specific rules about terrain placement for TNT, I would say that it looks plenty dense overall. In my opinion, most of terrain placement is a matter of taste. Obviously the real world is full of wide open fields with nothing in them, and cramped urban environments with countless nooks and crannies. So depending on where your adventure is taking place, I'd just aim for something that feels about right.
With that in mind, terrain density often offers advantages to certain play types. Want to take long shots with a sniper rifle, or launch missiles with a large area of effect? That is going to be useless in tight confines. Want to hack someone to pieces with an axe? That is a bad call if you have to cross wide open spaces to reach an enemy with a gun. A good mix, as I think you have in your pictures, will help everyone have fun and make some tactical decisions along the way.