Suicide Squad had humor but it was plenty dark. I mean, come on, it's a group of anti-heroes (if you can call them heroes in the first place). We've yet to see a DCEU film (other than Green Lantern maybe, if you call that) which serves up a straight-laced good-guy style hero.
That's why I said that they are 'mixing in' humor. I don't think anyone doubts that Justice League (the movie about fighting Darkseid) or Wonder Woman (the movie about WWI) won't continue the consistent darker tone of the DC movies.
Compare the trailers for Justice League/WW/Suicide Squad to the trailer for Man Of Steel. Suddenly there are jokes all over the trailers. WB marketing is pushing the humor in the trailers for a reason.