It's not like DC don't try to do lighter stuff in their main DCU line. It just doesn't tend to sell that well. Blue Beetle, Booster Gold and Power Girl are all examples of lighter, more fun superhero books that are well received critically, but don't set the charts alight. Blue Beetle got cancelled after being given a bit of a pass for a while based on critical response and the character's PR value as a minority hero, Power Girl's sales have dipped a lot since the debut, and while I don't know exactly what numbers Booster Gold does, I think it's supposed to do okay, but is not a top seller and isn't really something that would leave a big whole in the DCU if it got cancelled like a Justice League title would.
and the idea that ANY mainstream in-continuity comic is at the level of watchmen is beyond preposterous. even in the case of something like siege and the ripping of ares in half. A) that's the only thing of that level in the whole series. and B) it could be argued that the brutality of that was used for a very real and specific story driven purpose (as opposed to just being gore for the sake that it's "cool looking").
but even with that, siege doesn't come even close to being as "adult" as watchmen.