I've never been excoriated by a Methodist, or any other mainline protestant, for using the word Protestant to label their flavor of Christianity... .
Though, as I stated, I'm no religious scholar, and beyond the superficial and historical aspects I've noted here, I think you'll have to read the Book of Mormon and decide for yourself whether you want to call them Protestant. But take my caveat seriously: they don't like it, and they'll let you know it very quickly and very clearly.
Angus, I'm noting that the label "Protestant" really has very little meaning, because it would either include all Christian Churches founded after 1500 (actually earlier with the Brethren), or it would mean only those Churches founded as a result of the Reformation, which would exclude Methodists, Quakers, Swedenborgians.