The title is pretty self-explanatory and I could leave it at that: please, stop using mental illness as an insult.
But since my invitation would probably be ridiculed as a prime example of political correctness quite literally gone mad, I feel the need to elaborate further.
In English (but also in other languages, like Italian) there’s an impressive number of ways to pejoratively define someone suffering from a mental disorder or places and situations related to mental illness: crazy, lunatic, psycho, nut-job, whacko, freak, basket case, loony bin, bughouse. And the list goes on.