I don't know if the goddess who curses Barbara necessarily needs to be an evil deity. Just one she unwittingly offends in her hubris.
We do have the Olympian goddesses being benevolent to the Amazons even though they all have nasty track records in the myth (Athena in particular I don't think cared for the Amazons period), so giving Mafdet more of an edge wouldn't be hurting much. Especially as the fault could still be on Barbara Ann for trying to appropriate power she feels she's entitled to, and so Mafdet does a "be careful what you wish for" after she desecrates a tomb or something.