I read and hear these dismissals of Mormonism with some amusement -- because everyone who makes these charges holds beliefs and/or practices that outsiders consider just as irrational.
Let's begin with the religious critics.
There doesn't exist a religion without such beliefs. I say this as a believing and practicing (non-Orthodox) Jew, so I'll begin with my own religion.
I believe the Torah is a divine book. I believe that God took the Jews out of Egypt and that He gave the Ten Commandments to Moses on Mount Sinai. To atheists and secularists, these are not necessarily rational beliefs. And they are certainly not scientifically provable. As for practices that many would consider irrational, traditional Judaism has quite a few. Just to cite one: Orthodox Jews believe that they are not permitted to drink wine or grape juice poured by a non-Jew.
Concerning Mormon undergarments, it is worth noting that Jews have worn a "sacred undergarment" for thousands of years. Those who belittle Mormon undergarments might as well belittle the "fringes" (tzitzit) that observant Jewish men wear inside or outside their clothing. Yet, neither the Jewish nor the Mormon practice is in any way irrational. Wearing a garment to remind oneself to always act in a morally elevated manner hardly constitutes irrational behavior.
As for Christianity, non-Christians cannot be expected to regard the belief that God has a son who was born of a virgin as reason-based. (If they did, they would probably be Christian.) Nor do outsiders consider rational the Catholic and Eastern Orthodox belief that the wafer and wine consumed during Communion literally become the body and blood of Christ.
As for Muslims, the belief that the Koran was dictated by an angel to an illiterate man (Islam holds that Muhammad was illiterate) is surely not rational to a non-Muslim. Nor are myriad post-Koranic beliefs such as the requirement that women wear a veil.
If all religious beliefs were dictated by reason alone, there would be no meaning to the word faith. A healthy religious life is composed of both faith and reason. And so is a healthy moral life -- no non-Jewish rescuer of Jews in the Holocaust did so solely because of reason.