This is a deliciously beefy question! Thanks for asking!
The answer is a tripartite combination of perspectives. Through the eyes of the main character, the first perspective, the argument is completely valid. She is a woman suffering from an extreme case of post partum depression. Although it is arguable to say that she is not in a "right state of mind", it is also safe to affirm that (within her circumstances) her fears, tribulations, and anxiety are to be considered as true and worthy of our compassion. She actually sees these things. She really feels them, and she really accepts them as true. Therefore, she is suffering, battling and trying to control a situation that is entirely true to her. We are not in her shoes, so we could never have a right to say that she is wrong.
The second is the perspective of the secondary characters such as her husband, her doctor, and such. They are the ones who truly take her for a mild case that can fix itself. They do not consider her needs, nor know what the causes are for her behavior. Hence, they are the lesser perspectives as they really do not step up to try and solve the problem. Hence, she is still left out there with no other choice than to continue her path toward insanity. The secondary characters simply do not have a clue.
The third and final perpective is that of the reader. We could sit and say she is crazy, depressive, and dellusional. We know that there are no beings transforming in the yellow wallpaper. Yet, we are witnesses of her pain and, as a society, we must understand that her pain (despite of her hallucinations) is real. Her desperation is real, and so is her lack of control. Hence, if we look at the question from a deeper point of view, her argument would be completely acceptable.
It would take a very shallow mentality to not believe what she has to say. This is because we are expected (as readers) to cathartically and vicariously experience the feelings and experiences of the main character. Therefore, we are there with her in her struggle. As a result, we should agree with the main character in that her worries and fears are worthy of attention and that, in her mind, they are real, indeed.