You can be both, there's a reason Minsc doesn't make a Boo joke over the broken corpse of Khalid
I feel like there is some major misconception about grimdark. Realism or seriousness is not grimdark. Yes, Forgotten Realms do have adult themes, and seriousness, but also has silliness and levity and joke characters and comic relief. It is classic fantasy at its cheesiest, very far from grimdark. That being said, this game is already way darker and more adult than any Forgotten Realms game that I have played.
