Migration assistant won't use iCloud as a source and there is no such thing as an iCloud Backup of a Mac. However a Mac can sync some data with iCloud and anything synced with iCloud on the old Mac can sync with the new Mac simply by turning on the same sync options in the new Mac. You can use an internet browser and log into www.icloud.com to see what, if any, synced data is there.
Apple offers iCloud Backup for iPhones and iPads but not for Mac computers.
It is possible to sync data from a Mac to iCloud if you turn on those options (e.g., iCloud Photos, iCloud Drive including the Desktop & Documents option). Syncing has some of the benefits of a backup (and the benefit of being offsite) but is not the same as a backup. Major differences include the fact that not everything on your Mac can be synced and the fact that if you delete something, accidentally or otherwise, there is only a limited time period (30 days in most cases) during which it can be recovered.
Also see the Apple article: How to back up your Mac - Official Apple Support
And more information on syncing in:
Set up and use iCloud Photos - Apple Support
and
Access and manage your files in iCloud Drive - Apple Support
and
Add your Desktop and Documents files to iCloud Drive - Apple Support
For iCloud sync options on a new Mac running Ventura, go to Apple menu > System Settings > [username] > iCloud and click the "Show All" button in the Apps Using iCloud section.