It is possible to consider insanity the only form of freedom she could undertake in her societal position. Women were held to very specific expectations and any sign that they were reacting differently was considered hysterical--from the Greek, meaning something had gone terribly wrong within her uterus and was affecting her mind.
Modern doctors would diagnose the main character as suffering from postpartum depression or similar. The typical treatment now would certainly not include being locked away in a room, but at the time, this isolation was considered key. Women were forced into solitude during pregnancy and, in this case, thereafter.
Sinking into the manifestation of her mind allowed the woman to "escape" the prison of her bedroom walls. Her methods were unconventional--clearly evidenced is the horror her husband feels at realizing she has lost her mind, which is socially shameful--however, she was no longer "imprisoned" by the torment of the wallpaper or the enclosure of her room, or even the burden of her own depression. She no longer had to suffer the expectations and restrictions of others, and instead limited herself alone. You might argue that by setting her own limitations, she is made free--because she is the only person in charge of the "prison" she now occupies.