Miis can get depressed. Either through rejection of love, rejection of an apology after a fight, or a break up. Certain instances, such as after a break up, will make trigger both Miis in the relationship to become depressed. Instead of the usual Happiness bar, a Sadness bar comes up.
Happiness depends on ourselves! I suggest that you feed them their super / all time favorite foods. If you don’t know what they are, you should feed them something different every meal so that you will eventually discover what their super/ all time favorite foods are. Their favorite foods are more likely to be in the same category of food that they love.
Another great choice is to let them take a vacation by giving them travel tickets.
Other than that, baths, music boxes and swings travel tickets are also good supports. Clothing and hats aren’t so great unless you know what kind of style that they like.
Hopefully this helps in curing your Mii from depression!
I’m going to end this post with an explanation of a depression difference between the North American and Japanese version.
The North American version and Japanese version is different in the way depression is visualized. For example, In the North American version, the apartment icon turns rainy while sadness is represented by poor souls floating around the Mii.