Peppermint, at least, has some antimicrobial-and-other-single-and-small-cellular-life properties that would probably harm fermentation by risking prematurely killing the yeast colony, so if you're working with whole mint you probably want to infuse it in after the fermentation stage. I don't know about other varieties - types of mint that have less intense mint flavor may have similarly less potent oils and could be safer to mix in during fermentation, but probably best to do it during aging. If you're working with pre-extracted essential oils or flavor extracts, it'd be easy enough to just blend that into the completed mead during decanting and bottling. I'd be careful with the amount, tho - I've yet to run into a non-cream based mint-flavored alcohol that tasted very good.