Something that is light-hearted is intended to be entertaining or amusing, and not at all serious. There have been many attempts, both light-hearted and serious, to locate the Loch Ness Monster
Someone who is light-hearted is cheerful and happy. They were light-hearted and prepared to enjoy life
carefree and happy and lighthearted; "was loved for her blithe spirit"; "a merry blithesome nature"; "her lighthearted nature"; "trilling songs with a lightsome heart"