Are Mormons Christians?

No.

Mormons do not believe that Jesus Christ is God and God is in Three Persons (Father, Son, and Holy Spirit).

This shows why so many Mormons oppose Donald Trump.  Look at Mitt Romney, who is a Mormon.  Mormon Romney rather have Hillary Clinton as US President than Donald Trump.  Yesterday, he attacked Trump, trying to hand-deliver Clinton victory.  See this video:

Mitt Romney is a Mormon and Mormons are not Christians.  Thus, it is not surprising that Mormon Romney does not want Donald Trump, who is a Christian, becomes US President.

Why is Romney so eager to attack the official Republican nominee for US President?  He wants Hillary Clinton to win, because as a Mormon, he thinks that Clinton will be better for Mormons than the Bible-believing Trump.

Cultic groups like Mormons do not like the Holy Bible, which says that Jesus Christ is the only way to salvation.  Mormons do not follow the Holy Bible. That is why they have the Book of Mormon, which true Christians reject.