Sometimes comedy is made all the more sweet by the seriousness of a situation.
Watch Braveheart - that is a deadly serious movie, and yet there are moments, even when it is most serious, where there is a bit of comedy - it relieves tension.
Such as during the first battle, when the arrows are flying, and they are hiding behind the arrows and the irishman quips - 'The good lord has arranged to get me out of this one. But as for you, you're pretty much f*&#ed'.
Of course, you don't want to overdue it - but they don't call it comic relief for nothing. The campaign that had the most fun with that was actually my Ravenloft campaign - described elsewhere - where you had one party member cursed to turn into a fish-man, another cursed to turn into a zombie-lord, and yet another one slowly turning insane. At one point, they ran into a cursed NPC who was traumatized by it and told them about it to get both sympathy and to threaten them. The quick reaction of the party, which was unexpected, was the fish-man (Aqua-man as he became known) threw back his hood and showed his scaled head, the zombie lord threw off her cloak, and the stepped forward, pointing at themselves and said something like 'You think YOU have problems, Listen...'