I suggest a thesis that focuses on the oppression and destruction of the female characters that originates in the male characters' faulty perceptions of the female characters physical, mental, and/or social make up. You may also consider tying in the irony in both of these stories.
Maybe something like:
The oppression and destruction of the protagonists in "Desiree's Baby" and "The Yellow Wallpaper" originate in the male characters' inaccurate perceptions of the protagonists. The men in these stories eventually ruin the protagonists based on faulty assumptions related to the protagonists' physical, mental, and/or social stature.
Or, to tie in the irony:
It is ironic that the protagonists who are oppressed and destroyed in two short stories, "Desiree's Baby" and "The Yellow Wallpaper", are really not the ones who are flawed or otherwise abnormal.
In both stories the female characters' self-worth and self-fulfillment are linked to the husbands. Both Desiree and the nameless narrator of "The Yellow Wallpaper" struggle for a place where they can fully belong; their voices in life are repressed by overbearing husbands whose cruelties force the women's minds to change their states of being.
So, you could construct a thesis statement about the repression of the women which leads to their loss of self-worth and self-fulfillment.
Hope this helps! You can also check out some of the discussion questions and the q-and-a responses in the group sites for these two stories for additional ideas.