Can I backup my Dell PC to iCloud?

Apple offers iCloud Backup for iPhones and iPads but not for Mac computers (or Windows computers).

It is possible to sync data from a Windows computer to iCloud if you use iCloud Drive and/or iCloud Photos in iCloud for Windows.

See: Set up and use iCloud for Windows - Apple Support

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 computer 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.

And more information on syncing in:

Set up and use iCloud Photos - Apple Support

and

iCloud Drive FAQ - Apple Support