Why not? Stannis is the best man westeros has ever seen. Lyanna would be lucky to have him.

Best men usually don’t regularly perform human sacrifices for a god they don’t believe in, nor do they try to sacrifice their own nephew, or murder their brother, or neglect their wife and child. That’s not to say that Stannis is irredeemable, or the worst dude in the series, but he’s no angel nor is he the catch of the season.