General opposites:
* Serious
* Somber
* Grave
* Solemn
* Heavy
* Gloomy
* Melancholy
* Depressing
* Dour
* Grim
More specific opposites:
* Humorous: If you're talking about the lightheartedness of a joke, the opposite could be serious, dry, or deadpan.
* Carefree: If you're talking about a lighthearted attitude, the opposite could be worried, anxious, or stressed.
* Whimsical: If you're talking about lightheartedness in art or writing, the opposite could be realistic, practical, or grounded.
Ultimately, the best opposite will depend on the specific context and meaning you want to convey.