Byron's moral reputation left a great deal to be desired. Why is this ironic when compared to the final 3 lines of "She Walks in Beauty"?
This is an interesting way to approach the poem "She Walks in Beauty" because the issue seems to be raised in the poem itself. In the first stanza, Byron describes the beauty of a woman made of both darkness and light. This description applies not only to her physical looks, but to her soul as well:
And all that's best of dark and bright
Meet in her aspect and her eyes
What has caused the darkness, we can't tell. Though, if the story about Byron's cousin-in-mourning being the inspiration is to be believed, the answer is experience and despair. If this is the answer, it does not jive with the final lines. She may have spent her days "in goodness," sure, but if she has lost someone she loves, it is hard to believe that she would be either "at peace" or "innocent."
As you say, the insistence on innocence and goodness as the cause of the woman's beauty is ironic when compared to Byron's reputation. He was neither good nor innocent when it came to romantic entanglements, though he could be described as both dark and light. Perhaps what he sees in this woman is what he wishes for himself: a return to innocence, while still retaining the dark beauty of all his experience.
I like the assertion being made in the question. It is very insightful. When Byron was described as "mad, bad, and dangerous to know," it is something that can be seen in stark juxtaposition to what is offered in the poem's concluding sentiments. The interesting thing is that while Byron himself was one of these individuals who was far from being "at peace" with anything. The type of lifestyle where moral impurity was present was seen as almost part of the "package" with Byron. Yet, he seems to be making an assertion of not only physical beauty with the closing lines of the poem, but one of moral purity and a sense of ethical conduct that is above reproach. If one were sympathetic to Byron, perhaps the closing lines are a desperate plead to hope for something that is not present in his own life, akin to an alcoholic yearning and speaking for a life of sobriety. At the same time, if one were unsympathetic to Byron's lifestyle and his exploits, then the call and claim for moral purity is something that is beyond hypocrisy, in that it is extremely unfair for him to make claims of purity present in his "other" while not even attempting to adhere to such claims in his own consciousness.