She was picked to get the party's base excited, because up to that point they were as animated as a brick. Did she cost McCain votes? Maybe, maybe not. It's one of those tough things to argue whether a perceived "shitty" running mate actually costs votes.

Consider this: Dan Quayle quickly was seen as a joke in 1988, getting creamed in his own V.P. debate by that certain moment that's one of the most replayed highlights in the history of televised American debates. That said, Bush Sr. still won that election in a landslide.

Or 1952, Nixon was accused of doing illegal shit* and people (some within his own party) called for Ike to dump him. He gave the Checkers Speech, rallied his party's base to support him, and Eisenhower won in a landslide.

*=The same sort of shit that the Democratic candidate for President that year, Adlai Stevenson, had done as well but without any public attention what so ever.