I think it was excellent for Sarah Palin's image to laugh at herself on SNL. Tina Fey's impression of the VP candidate is a fixture in the election and the fact that Sarah Palin got in on it was brilliant for her. Obviously viewers agreed because that SNL was one of the most viewed episodes.
While her appearance may not have earned more votes for McCain, it was a great image booster for their campaign. They probably felt the need to compete with the hip crowd that Obama appeals to.