I don't really see it that way.
It depends on the show itself how much humor will be a factor. The Stargate shows (maybe apart from Universe) were pretty light and funny at times which was part of their appeal, Firefly was funny as hell and i think many other shows which i don't watch have occasional one liners or funny scenes to break the mood.
TNG was "serious" Sci Fi.. it tried at least and did its best to be taken seriously. They had their fun moments (especially with Q as already mentioned) but overall it was pretty serious.. sometimes too serious if you ask me.
DS9 had a better balance.. it was still serious at times, sometimes pretty dark and grim in the later seasons but they didn't forget the humor and levity and sprinkled it in between freely or outright made the occasional fun episode.
With shows like BSG or Dark Skies the theme itself is a bit counter-humoristic. BSG has humans on the verge of extinction after a genocidal war which they lost.. not much room for fun when you're constantly on the run from your enemy and mistakes cost lives.
Dark Skies has Earth taken over by hostile aliens, civilization as we know is shattered and the last free humans are in a constant struggle for safety and ressources.
Earlier SF shows didn't have such dark themes.. they were "just" adventure shows set in space and as such could be written lighter and with more fun in mind. So perceptions can be misleading and i think current SF is pretty much ok if not better because it has much more variation to suit more tastes.