I'm not so sure I would have called Chrono Trigger lighthearted, though. From the first time I stepped into 2300 A.D. to everytime I step into it thereafter, I always feel quite affected emotionally. My heart literally hurts whenever I have to walk through that depressing, destroyed world. Then there was the destruction of Zeal, which both pissed me off and depressed me even more. Of course this might be more due to my overactive imagination and tendency to overreact emotionally to everything, but I'm not so sure it was all that lighthearted.
...then again...given Marle's underreaction to Crono's death...as well as her actions in the ending where Crono is still dead...it certainly has lighthearted streaks.