How does "She Walks in Beauty" reflect or differ from what Byron believes about the woman?
My answer to this intriguing question is another question: how could it possibly differ? As I read this poem this is Byron being his most sincere. I detect no possible sarcastic or satiric undertones in this quite restrained (for Byron,) lovely poem.
All the poetic images and allusions point clearly to the woman he describes being every bit as mild, calm, and beautiful as a starry night. That he would choose the night to compare a beautiful woman (the subject was presumably a young dark-haired woman wearing a dress with shiny spangles on it) is a bit self-conscious (compare Shakespeare's "Shall I compare thee to summer's day"? , Sonnet 18, or "Juliet is the sun" Romeo and Juliet Act 2 Scene 2), but it is nevertheless apt and very effective. Certainly a starry night can be every bit as beautiful as a sunny day -- so why could he not compare a woman to the night (which is a standard tenet of Asian philosophy, the dark being female, but I'm not sure Byron knew this)?
The discussion of her outward beauty progresses away, in a most uncynical fashion, to a paean to what Byron believes what must be her complete and inherent goodness of soul.
The smiles that win, the tints that glow
But tell of days in goodness spent
A mind at peace with all below,
A heart whose love is innocent.
This particular brand of gentle, unshowy beauty, Byron argues, must mean that she is of particular innocence, purity, and goodness. He expresses no desire for her, or speaks of anything social about the woman except her uprightness. He admires her both like a great work of art and as a (secular) saint -- as a pattern for both outward and inward beauty. It's possible that it could be argued that the historical Byron mistreated women so badly that he couldn't possibly have felt such pure admiration for any woman; the poetic Byron expresses just such an emotion very eloquently.