The Catholic church says that Mormons are not Christian, you use the word heretic to describe Christians, something is not adding up.
They certainly are heretics.
They claim to be Christian but have unsound doctrines - even very unsound doctrines, and they use both the Christian scripture and the concepts threin. Thats a heretic.
Back in the sixteenth century we would have been prosecuting them as heretics, open and shut case.
Are they not Christian ? I doubt you will find such a blanket statement from the Catholic church, as Christian is both a theological concept and a linguistic one. All we have here is a statement saying (or broadly implying) that they are very heretical.