The point where Mars could be some sort of viable backup of anything on Earth is centuries and trillions of dollars away. Up until a Mars colony(ies) has enough local infrastructure and industry to be entirely self-sufficient it's survival will require access to Earth's infrastructure and industry.
Mars also has fewer natural protections than Earth: a thinner atmosphere, no magnetic field to speak of, and no giant moon that deflects or absorbs at least some percentage of objects that would otherwise impact Earth. So Mars is more susceptible to dangerous radiation and object impacts than Earth.
For the same cost outlay (or way less) as trying to make Mars an effective backup for Earth Earth could be made more robust and dedicated purpose-built backups could be created.
Mars as a backup of Earth is like a cheap eBay thumbdrive hanging from a wind chime on your patio is a backup for your data.
Mars also has fewer natural protections than Earth: a thinner atmosphere, no magnetic field to speak of, and no giant moon that deflects or absorbs at least some percentage of objects that would otherwise impact Earth. So Mars is more susceptible to dangerous radiation and object impacts than Earth.
For the same cost outlay (or way less) as trying to make Mars an effective backup for Earth Earth could be made more robust and dedicated purpose-built backups could be created.
Mars as a backup of Earth is like a cheap eBay thumbdrive hanging from a wind chime on your patio is a backup for your data.