Mopiest is the superlative of mopy or mopey—the most dejected, listless, or glum among a group: She was the mopiest kid at camp after her best friend went home.
The word suggests sulking or low-spirited brooding rather than acute grief.
1. emotional — gloomiest, Most dismal or despairing; saddest (Superlatively sorrowful); sulkiest (Most inclined to brood)
2. comparative — gloomier, Increased in bleakness; sadder (More sorrowful); superlative (Highest-degree word form); extreme (Furthest from moderate); ultimate · most · ranking · bottom · lowest · darkest · bleakest · dreariest · dimmest · grimmest