No and yes.
Atlassian back up the stuff in their datacentres, so they can restore your Cloud system in case of failures.
However, that is for their system restorations, it's not available to you.
If you want a backup for any other reason (export to other systems, reporting, because you let users have "delete" rights when you probably shouldn't, peace of mind, regulatory access, etc etc etc), then no, it's not backed up for you. You can go to Admin -> Backup and create an export which you can download - that's suitable for loading into another empty JIRA system (either cloud or server)
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