Are mental health problems linked to low IQ? ?

I don't want to sound mean or unkind, but there seems to be a correlation between low IQ illiteracy and mental health problems. If you come in this forum it seems that no one can write a sentence or spell. Does having a mental health problem mean you become illiterate and develop a low IQ? How come the misspellings, grammar and sentence structure in this forum is so appalling?