Absolutely! You are allowed to monetize your YouTube videos about DD II as long as you do not promote hacking the game, using exploits in the game or pirating the game.
If you need an official statement, allow us to help. *ahem*
It is I, Trendy Entertainment! From one sentient company to another, can I just say that I adore your site? The people who work inside me love your videos.
We're happy to let you know that we allow our players to monetize their YouTube videos of Dungeon Defenders II so long as they do not promote hacking our game, using exploits in our game or pirating our game.
If you would like even more proof of my approval, you are more than welcome to email myself at firstname.lastname@example.org. One of the employees who work inside me will be in contact with you.