There is no evidence in the play that would support the claim that Amanda suffers from any kind of condition. However, there is enough proof that shows that Amanda lives under a severe case of self-denial and unfinished businesses. That, in itself, is not indicative of a medical condition, nor of a psychological disorder, but mostly of a social inability to cope with change and reality.
The way she nags and continuously talks are only the result of a mind which is restless, not sick. She can certainly see that there is something wrong with her daughter and son, and the fact that they are living on dire straits does not help her much either.
Furthermore, Amanda comes from a Southern family with means who taught her the ways of the Southern belles, and got her accustomed to the finer things in life.
Now she is a woman living in a small apartment in the city with two grown, ineffectual children, no money, and abandoned by her husband. She sells magazines to try to make ends meet to no avail. To top it all, she gets the sad reality check that her daughter, who limps and has extreme social anxiety, will never have her dream-guy, Jim, as her beau.
Amanda is sane enough to recognize what is missing in her life, and in her children's lives. She simply has to bottle it up and pretend that everything is OK, or that everything will someday be fixed. Far from this being a positive view of life, it is actually a suppression of reality leading towards denial. Denial is not something that a person with mental illness is able to feel, because it takes differentiation and analysis. This is exactly what the family in this play is experiencing: The sad reality of seeing that their reality is nowhere to where they wish it were.