Can I use iCloud to act as a time machine?

iCloud is a syncing service, not an external hard drive. You should consider having at least 2 backups on 2 separate external drives as hard drives do fail. Time Machine will back up to multiple drives, alternating between them. Another often recommended backup is one that is stored away from your home in case of fire or theft. Since that one isn't backed up daily, I'd recommend a clone. There are also cloud based backup systems you can use for off site storage.

 Time Machine Basics

Time Machine Basics (2)

Clone  - Carbon Copy Cloner          (Often recommended as it has more features than some others)

 Clone – Data Backup

 Clone – Deja Vu 

Clone  - SuperDuper

Cloud Backup Services - Best

Cloud Backup Services - Best (2)

In addition to Eric Roots answer:

iCloud is used to synchronize . This means that if you e.g. delete a file on your Mac, it will also be deleted in your iCloud.

Here is how to set up iCloud: https://support.apple.com/guide/icloud/set-up-icloud-mmfc0f1e2a/icloud

If you want to back up: use TimeMachine

Here is how to use TimeMachine: https://support.apple.com/en-us/HT201250

Depends on what you mean by "as a time machine"

Time Machine can't use iCloud as a place to store its files.