How to add notes to iCloud via sending an email
Although I, personally, do not use the Notes.app (preferring Simplenote) my Wife uses the Notes.app in conjunction with her gmail account.

So if you have a gmail account it has a notes label that will add emails with that label to your notes. 

It should be easy enough to create a rule to route any emails to that address with a specific phrase in the subject line to mark it as read and with the notes label.

This is not exactly what you were looking for. It requires a gmail account which may be a no-starter for some people, and I don't think that Apple has a way to create a note from an email other than copy/paste. But it might be worth looking into.