I would say NO.
*woohoo!* I get to be the nerd who says, “FIRST!”
Anyway, it can be a chemical imbalance in the brain.
It can be caused by situations that are either immensely stressful or have broken the person down over a long period of time.
I just don’t see it as a mental illness.
That link says it’s a mental disorder. Which means it’s out of the ordinary. It isn’t a disease because there doesn’t mean there is something wrong with your brain necessarily.
So I guess the answer technically could be yes. Since an illness can be cured & depression can also be curable. Still, I don’t like to label it that way. It sounds too harsh & negative.