There is a difference between being non-Christian and being anti-Christian, IMO.
Mormons are anti-christian.
I'm not sure i see a meaningful difference between a Mormon and an agnostic. How is one better than the other?
The only Mormon libertarian I'm familiar with is Will Grigg. He's not likely to ever run for office, he's an LRC blogger who focuses on police abuses who happens to be Mormon. If he were to run for an office, I wouldn't hesitate to vote for him because he's Mormon. Nor would I hesitate to vote for a Catholic libertarian like Tom Woods or Judge Napolitano.
Now, the lesser of two evils is always evil and should be rejected. But, in a political role, I don't see someone who is non-Christian yet has the right policy positions as evil. Note: I understand that they are themselves morally evil. But putting them in a position of power isn't evil because by doing so, you are really voting for good policy. (This is doubly true in the United States, perhaps less true were we voting for an absolute monarch: since at least in theory the American system is of law rather than men, and the kind of candidate that does not realize this isn't even qualified at the most basic level anyway.)
Now, given a choice between a Reformed libertarian and a Mormon libertarian, I would choose the former, barring any other extraordinary factors. But, I rarely get to be that picky. As it is, if I have the option to vote for Rand Paul (who is Presbyterian BTW: though not exactly sure what kind) rather than being left with no real options, I'll be happy. The idea of even getting one libertarian purist to choose from seems far fetched, let alone more than one.
I can't stand Romney though. He hates freedom.