Mormonism appears more ridiculous because of its relatively recent creation. Modern humans tend to think of religion as something intrinsically ancient and traditionalistic, and therefore have problems with "New" religions like Mormonism and Scientology.
Contemporary Christians are reacting to Mormonism much the way Jews reacted to Christianity in the years following the death of Christ. Because of the recentness of the events, Jews were skeptical that Christ could be anything other than a charlatan or a rabble rouser. Much the way establishment Christians today view Mormons.
However, to me, it's essentially a situation of you claiming that your imaginary friend is blue, and Mormons claiming that he is red. And you think they're nuts because you've been saying he's blue since 20 AD, but they've only been saying he's red since 1830. It's really quite an arbitrary distinction, but I don't expect you to acknowledge it.