Can I use time machine to backup to iCloud

As muguy has already mentioned, iCloud is basically a syncing service as far as the Macs are concerned. The best method to transfer data between your old and new Macs would be via the Migration Assistant from a Time Machine backup. This backup would need to be on either a locally attached or network drive ... but, sorry, not from iCloud.

You cannot. You can sync files in iCloud Drive, and also enable syncing for desktop and documents, but you cannot back up a Mac to iCloud. Time Machine backups are made to an external drive; ideally you've been doing this all along to back up your data.

Do you have a Rescue Plan In-Place. This comprises having 3 Backups using 2 methods and 1 Off Site incase of Theft, Loss or Natural Disasters. A Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS. 

For addition purposes - two Tested Bootable Clones each to separate external drives. This is insurance incase the upgrades goes sideways. At least one Tested Clone and / or Time Machine Backup should be Off - Site

Manage your iCloud storage

Set up iCloud on all your devices

Back up your Mac with Time Machine