Speaking from experience doing a comic that was originally mainly to be a lighthearted gag-a-day comedy series that then turned into a more 'slice of life' comic with serious moments, I'm fine with both if done right and they meld together well. (Option number 3)
My comic, POW! Right in the Nostalgia, still does a lot of lighthearted/comedy stuff for the most part, but I also throw in serious parts through story arcs I do time to time to help flesh out my cast and make them more human.
For example, my main character Charlotte (short blonde haired girl with green eyes), is most often shown as being a big nerd for retro stuff and enjoys excitedly talking about the stuff or showing it off to friends and her fiance's niece with usually comical results happening.
However, in the story arcs, I've also shown she also has a seriously strained past with her parents that has came up many times over the course of the comic's almost 3 year run that caused her extreme anger and lashing out at them even being brought up, and now am doing an arc where she finally confronts these issues.
I really think the right mix of lighthearted with serious helps kept readers both on edge as well as give them brief sighs of relief.
Don't just have everything all rainbows and unicorns one panel and suddenly demons brutally slaughtering everything in gory fashion on the next basically.