There is plenty about Michele Bachmann to give one pause. But, the fact that she gets migraines is not one of them. And, as some commetators have noted, it is difficult to imagine a similar concern being voiced about a man: There is some sexism at work in this, playing to a stereotype that is palpably false. It works against Bachmann only because she is so little known. Can you imagine anyone being disturbed to find out that Hillary Clinton had migraines? Of course not.
Historically, some of our greatest presidents have suffered from serious medical conditions. FDR was a great president, and he was also confined to a wheelchair. JFK was on so many pain-killers it is remarkable he was able to get through the day, but he did alright in the Oval Office.
The fact that Bachmann suffers from migraines is not an issue.