Joseph Smith & Co. were not heretics from Catholicism before they created Mormonism because they weren’t heretics as Protestants. Can we say they are heretics from Protestantism? Possibly. As far as I’ve read, Smith was raised by his parents to read the Bible and believe in Jesus, but they weren’t all that religious. He joined a Methodist church, but that was short-lived, and who knows what he really understood?
People say Catholicism is wrong, the whore of Babylon, the beast of Revelations, etc., that doesn’t mean they have an orthodox understanding. I’ve never read anything of Smith’s that indicates he understood orthodox Christian teachings. Sidney Rigdon was a Christian pastor before joining Smith to form Mormonism and helped formulate the Mormon view of God as three distinct beings. I think there is a stronger case to claim Rigdon was a heretic from Protestantism than there is for Smith.
Proclaiming a person is an unrepentant heretic is claiming they are damned. As I said, I leave judgment of individuals to God. The religion itself is of course heretical in its teachings.