How did Mormons react to Susan B Anthony's letter?

Susan B. Anthony wrote possibly hundreds of letters in her lifetime, so I would need more information about which letter you are referring to.

Women in predominantly Mormon Utah already had the right to vote and always had, so although they likely supported the suffragette position, they were not very involved in the process. Susan B. Anthony was aware of the women's voting rights in Utah and was acquainted with several prominent Mormon women in Utah.

Susan B. Anthony did write a letter that was published in the Salt Lake Tribune, asking the women of Utah to not vote for prominent Mormon polygamist B.H. Roberts to the US House of Representatives in 1898. However, after B.H. Roberts was elected and denied a seat in the House, (an act which most suffragettes supported) she went on record saying that it was unconstitutional to kick him out of his elected position and wrote him a personal letter of support.