iCloud back up

Notes (and other things such as Contacts and Photos) that you can view at iCloud.com are not part of iCloud Backup. If you can view them at iCloud.com, they are synced rather than iCloud Backup.Y ou can't view the contents of iCloud Backup at iCloud.com.

Data that you choose to sync isn't included in iCloud Backup but is included in iCloud Backup if you aren't syncing it. See the following from What does iCloud back up? - Apple Support

Any data on your device that isn't regularly synced is stored in your iCloud Backup.

If you are looking at iCloud.com and all of your notes aren't there, one possibility is that you don't have iCloud Notes turned on in one or more of your devices. See: Set up iCloud for Notes on all your devices - Apple Support

When you go to Settings > Your name at the top > iCloud > Show All > Notes > is it tuned on there?

Notes are not included in a backup, unless they are in the category “On My iPhone". They sync to an email account.

Then read FoxFifth's reply, which is correct.