Star Trek is about adventures and exploring space, not about diseases and mental illness. There are plenty of programs about such items in each and every channel for those who are interested.
Not to mention the various doom-and-gloom series who seem to be the trend of the recent times when it comes to series of all sorts. A lot to choose among for those who like such series.
If they absolutely want to turn Star Trek into a hypochondriac's wet dream, something about diseases and illness, there are more suitable candidates to be a victiom of such items than Kes who probably would be the last one to go insane on that ship.
And I still can't see the reason to bring back a character which was dumped during dubious circumstances and which they have put down a lot of efforts to make us forget, only to ridicule, humiliate and ultimately destroy the character, thus showing a finger in the faces of the fans of that character.