Well, it depends what you mean by mental illness. Are you talking about severe mental illness, such as schizophrenia or a psychosis...or are you including mental issues which are much more minor, such as anxiety and depression, under the definition of mental illness? In my opinion we all have mental issues of some sort, and most people will suffer from things like anxiety, fear and depression at some point in their lives, but such things aren't mental illnesses. I also believe that we all experience ups and downs of moods, and this is most of the time natural. Certain situations in our lives too, can bring up anxiety, fears or periods of depression and that too is normal. There are also people's personalities to take into account. For example just because someone is introverted by nature and likes to be alone doesn't mean they have a problem like agorophobia. It would be really hard to read a card as a mental illness, because it might just be that they're going through some issues of anxiety or depression at that moment in time, or that they're a neurotic and introverted person. I think any of the cards could show mental issues or illness, there wouldn't be any one card. It would be really hard to pinpoint what it was exactly though...if it was a proper mental illness, or something else.
I've seen the moon more as internal fears and anxieties, and of being frightened by what one can't see and panicking, almost an unwarranted fear of the unknown. I liken it to the feeling of being scared of the dark that you have when you're a kid, and running fast because you think things are chasing after you, but they're not, there's nothing there or it's not what it seems (probably a small thing casting a big shadow!). The eight of swords is almost like a mentally crippling feeling, feeling like you can't do anything, that you're trapped and immobilized. I have seen the 5 of cups as depression, and I keep wondering about the 5 of cups in the druidcraft...the way the figure is standing near the water's edge, gazing out over the water. Makes me wonder if they're contemplating suicide, or at least feel low enough to have thoughts of death and wonder what it would be like.
The nine of swords has often meant that I'm blowing things way out of proportion and thinking too much...overanalysing and making myself anxious over nothing. Worrying when there's nothing to worry about because of insecurities and low self esteem. I could also see it as insomnia and having terrible nightmares.
I've seen the 4 of cups as mild depression and a feeling of malaise, more from a feeling of being dissatisfied with life, and bored moreso than depression really. Feeling a little down because your life is so dull and boring, yet you're also in a state where you're too apathetic and lazy to get yourself out of the funk that you're in.