A cult is a religious organisation which follows the teachings / thoughts of a human, not of God. Mormonism comes into that category as they follow the teachings of Joseph Smith, not God.
The Bible is pretty universally established and accepted amongst the mainstream Christian religions (RC / Anglicanism / Baptists / Methodists / etc.) and the Bible has some pretty established teaching about the Trinity / Resurrection/ Jesus / etc. when you have a 'church' such as the Mormons teaching somehing different, then they are not a mainstream, Christian religion.
They have a lot to admire -esp. community and service, but they are not teachig orthodox Christianity, so to that extent are no different to any other organisation based on a human's teachings such as Scientology, etc.
The Bible is very clear on redemption through Jesus's sacrifice on the cross, and the need to personally repent / ask forgiveness / choose to follow Christ - passages, such as John 14:6 - I am the Way, the Truth and the Life. No one comes to the Father except through Me. is pretty self explanatory and completely denies the ability of organisations such as the Mormons to pretend that they can somehow 'get the deceased into Heaven' by baptising family members, it is totally non-biblical and wrong - God is absolutely clear that due to sin, we all have fallen short and will be denied Heaven and being wih God when we die, except for the death of Jesus on the cross taking that punishment, it is therefore only throuh that death, through accepting that death, through Christ, that we can be given access to Heaven etc. It is a personal decision and commitment which can not 'done' by anyone else...
so, sorry, but your mother has been taken in by the (deliberate) kindness of the Mormons and fooled into believing that it is a Christian church, it is not, never has been, and never will be...
Check everything against the authority of scripture, that is the only authority, and any chu4ch (CofE, RC, or others) should always be assessed against that authority - there are plenty of individual anglican and RC churches that get it wrong, but the underlying principle is still correct -to follow the authority of the bible.