I doubt that. McCain was trailing Obama until he picked Palin as VP. Within a week of picking Palin, he had surged into the lead. And then the financial crisis sent him plummeting even further behind than he'd been before he'd picked Palin. Palin definitely wound up not helping McCain, but I think McCain would have lost regardless of who his VP was.
2008 Presidential Election polls
It's hard to separate which hurt McCain worse: Palin's support evaporating or the financial crisis. It's hard to say a President's VP pick can do more than provide a temporary fluctuation in any event.
But the more people learned of Palin, the less qualified they thought she was:
More 2008 campaign polls. Unfortunately, you have to scroll down a bit to get to the polls that include Palin, but in early Sep, 45% considered her qualified to step in as President and 46% considered her unqualified. That early vote of 'confidence' eroded to a 40%-55% split in little over a month.
In mid-Oct, 80% of people thought McCain had the right experience to be President, 76% thought Biden was qualified, 49% thought Obama was qualified, and 43% thought Palin was qualified. While Palin lost credibility quickly, she wasn't perceived that much worse than Obama in terms of experience. Just prior to the election, 67% felt Biden had the right qualities of a President, 65% thought Obama did, 62% thought McCain did, and 37% thought Palin did.
Palin hurt McCain's campaign in the long run, but McCain was losing before he picked her and he wound up losing after he picked her. The initial surge might suggest the election was winnable with the right VP candidate, but VP announcements rarely result in more than a short term surge. McCain's lead would have disappeared even with a good VP candidate. The best Palin would have done would be to help (or hurt) a tiny bit. The financial crisis (and McCain's initial reactions to it) hurt him more than Palin.
But she definitely didn't help him.