Back up MacBook to icloud

No you can't back up complete Mac data in which system files from user library and system library and applications are also included to the iCloud , to back up Mac time machine is used see the article How to back up your Mac - Official Apple Support

The iCloud has a different feature , when an apple id is signed in system preferences and iCloud features are turned as on Change which apps sync and store data with iCloud - Apple Support

The applications sync and their data that was used saves in iCloud , but for back ups see the complete details What does iCloud back up? - Apple Support