I'm sure it doesn't *need* to be added. Is it a good idea to add it? I think so - it'll add confusion and insanity to the mix, along with humour , a dark twisted humour. If it was just wall to wall evil then I, for one, would think it sounds very dull. As it is, I want to buy this *because* of the whimsy.
Exactly. Wall to wall evil is no good. Same with wall to wall insanity, or darkness, or even whimsy. You NEED a good mix of themes to make a commercially viable adventure, film, book, whatever.
The game is built on three tiers - combat, exploration, and social. If you only have one overarching theme - "evil horror" or whatever - then it colours everything. And it means that the social and exploration tiers of the game can suffer as a result, because there's fewer "points of light" and fewer points where those methods of game input are practical.
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On to the point at hand - I'd say that, well, YES. Something needs to be added to a dark and depressing theme to soften its impact. Does it need to be whimsical or humour? No, not really. But something to break up the evilness is necessary. I've played games that encourage only a dark and depressing tone (say, Call of Cthulhu), and, while they work for a session, few people want to commit to a full campaign of it in my experience.
Compare that to games like WHFRP, a dark horror game with a heavy dose of whimsy, and a lot of people get really addicted to it in a campaign setting. Or a game like Shadowrun, which has a LOT of hidden jokes in the text and is a pretty funny game at times... despite a rather pessimistic and cynical tone to the game.
Contrasting two themes really helps accentuate an overarching tone. Or, to quote Woody Allen - "Tragedy plus time equals Comedy".