I had this come up recently - I was having a reading done by another reader, and we then had a bit of a discussion about Temperance and mental health issues.
Looking at the Gilded Tarot - my deck of choice the past year - I could see this in Temperance (if it was appearing in a negative since), there is such an extreme between the two cups - one showing intense fire, the other water - could represent to me the extremes between positive and negative thinking, which can lead to mental unrest.
The reader who was performing the reading, in the position in which Temperance appeared, saw a form of mental illness - not quite depression, but a tangent of it.
Temperance, in a positive position, or upright, I wouldn't read as a mental illness. But if it was reversed or ill dignified, I could potentially see it - depending on the situation.