The quote you have highlighted draws attention to the narrative form of the novel, and the way that Frankenstein is narrating his exploits and adventures to Walton, who is in turn setting it down on paper for our benefit. The comment above all signifies the way in which Victor is aware that his fate, and how he will be remembered, is in the hands of Walton, and how he desires to have control of how others will interpret what has done. Let us just examine the quote in full:
"Since you have preserved my narration," said he, "I would not that a mutilated one should go down to posterity."
Victor is forced to trust Walton to present his story in an unbiased way, just as we as readers are forced into this relationship of trust. Walton is entrusted with this story as one who himself is prone to the same grandiose desires of Frankenstein. However, Frankenstein wants a fair deal in terms of how he will be remembered. Although he recognises his many mistakes, at the same time he does not want to go down in posterity as a man who "played God" and did nothing about the consquences. He is eager to reinforce his image as a man who recognised his mistakes and did everything he could to rectify them.
In chapter 24, of Mary Shelley's Frankenstein, Victor reads the notes which Walton took on his life. After reading the notes, Victor quickly "corrected and augmented them in many places." When explaining to Walton why he was doing so, Victor stated, “Since you have preserved my narration,” said he, “I would not that a mutilated one should go down to posterity.”
What Victor is stating here is the fact that he recognizes that he went against the laws of nature in creating the creature. Given the fantastical story Walton is preserving for Victor, Victor wants to insure that people, when reading of the tale, come to believe what Walton is telling them. He wants Walton to be credible.
Victor, is not credible. As stated before, he went against the laws of nature and wishes that the world knows that he admits the fact. He, by making the statement about the creature not going down in prosperity, he wants the world to know that what he did was wrong and that he can fully admit to it.