In general the tone is bleak and realistic. I think the first hour or so is probably the worst, when in quick order you get introduced to the hollowborn children, ghostly memories of a past inquisition, ritual suicide, and the genocidal purge of a village currently in progress. After you get past the hanging tree, it calms down to Baldur’s Gate levels I think.
There are humorous moments though, especially through party banter. Some of the more cheerful companions include Eder, Sagani, Hiravias, Kana Rua though their personal quests can be serious matters. Aloth and especially Durance should be included in your party too. Even though they aren’t barrels of laughs they are well written and offer unique insights into the main quest.
But there is nothing of the silliness of the Divinity: Original Sin games or the dark humor of the Fallouts. Obsidian have said Pillars 2 will be, while still serious, not as depressing and grim as some players found the first one.