Just to confirm, you're asking about the book, and not the musical?

The amazing thing about Mormonism is that we can actually answer this question. I can't read the early critics of Christianity, but we can read the early critics of Mormonism and watch that movement grow into a new religion.

Like you might expect, there was a lot of ridicule and derision. So for example, we know about Abner Cole, the editor of the Palmyra newspaper. He published a satire called the Book of Pukei which made fun of the treasure quests and the new faith.

Pomeroy Tucker worked for the printer of the Book of Mormon. A few decades later, Tucker published a book detailing his skepticism of the movement.

By 1834, we see a "conspiracy theory of the Book of Mommon" that claims Sidney Rigdon masterminded a plan to steal and publish a deceased author's unpublished manuscript. It's pretty laughable, but it shows that as early as '34, even critics of the book often doubted that Smith had authored it himself.