She put my arm about her waist, (16)
The speaker is so passive and apathetic that he just lets Porphyria move his arms around like a doll.
I propped her head up as before, Only, this time my shoulder bore Her head, which droops upon it still (49-51)
By the second half of the poem, the tables have turned: now Porphyria is the passive one (because she's dead), and the speaker moves her body around as though she were a doll.
The smiling rosy little head, So glad it has its utmost will (52-53)
The speaker refers to Porphyria just as a "head" – an object, something that can't have a "will" of its own at all.