I think you are spot on with this idea. There is a definite sense in which the madness or the lunacy of the narrator worsens with the way that her husband and the doctors that are advising him require the narrator to remain ever-more cooped up and imprisoned in the room. Her husband, John, is of course doing what he believes is best for her by ensuring that she does not exhaust herself and is given lots of rest, but the result of this is a mental imprisonment that forces the narrator to focus even more on the yellow wallpaper and see her alter ego, a woman trapped, in its pattern. Note the following example:
I tried to have a real earnest reasonable talk with him the other day, and tell him how I wish he would let me go and make a visit to Cousin Henry and Julia.
But he said I wasn't able to go, nor able to stand it after I got there; and I did not make out a very good case for myself for I was crying before I had finished.
Note how it is John who tells her what she can and cannot do. She is unable to decide or given a voice in this decision. Thus it is that the mental imprisonment she experiences leads directly to her slide into insanity.
Thus a good thesis statement you could use might be:
The increased restrictions that are placed on the female narrator drive her ever further into insanity.