Does a iCloud backup work as a computer backup?

You would absolutely want an iCloud backup on your iPhone/iPad using iOS 17 before attempting to use iOS 18 beta. It is important to note that it is possible to still lose iCloud data such as Photos, Contacts, or anything else where you are using iCloud to sync your device. You should also know that there is no device support when using the beta software, so if you run into trouble, your only option is to remove the beta before support would be available. The iCloud backup may not contain all the data to restore your device. Review this support article to see what iCloud will backup:

What does iCloud back up? - Apple Support

It is only major if it happens to you, others will not find it a problem at all. I remember when iOS 9 beta was released, many users who tried the beta lost their iCloud data. You can archive your iCloud data and also backup your iPhone to a computer, but this requires a computer that you do no have.

Archive or make copies of the information you store in iCloud - Apple Support

Nope

Even if less than 1 tenth of 1 percent, you wouldn't want to be one of the unlucky ones in that group.