I think it's an emotional state which can really changed to a disease if you go a lot and more thorough in it or when it brings many consequences

Can be both

I think its a mental disease The emotional state is called boredom

It can be just an emotional state if it's just for a brief time, but clinical depression is a mental health disorder with persistent feeling of sadness or loss of interest that can lead to a range of behavioural and physical symptoms. These may include changes in sleep, appetite, energy level, concentration, daily behaviour or self-esteem. Depression can also be associated with thoughts of suicide. The mainstay of treatment is usually medication, talk therapy or a combination of the two.

100% emotional