Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Dark and Joyous
Like other Shakespearean romances, Cymbeline's tone jumps back and forth throughout the play. At first, the play is obsessed with death and punishment: the first two-thirds of the play are filled with jealousy, misogyny, slander, and the constant threat of death.
There's a lot dark and heavy stuff, and that includes mourning and loss. Belarius, Guiderius, and Arviragus, for example, believe Imogen has died, as does Posthumus; this leads to some of the play's more memorable lines—which are about the nature of death. On top of that, the Queen and Cymbeline mourn for the unexplained disappearance of their sons, and poor Imogen mourns the loss of her good name and of husband's faith in her.
In fact, the play is more dark and scary than it is joyful. It's only at the end, in the final act, that we get any glimpse of happiness for the characters. It seems like the joy that Cymbeline feels is so overwhelming that it makes up for the gloom and doom before it.
It's like Shakespeare is reminding us that the line between comedy and tragedy is very thin. One different move, and this play could have ended in a bloodbath.