What you're seeing with that wall of text is Mail.app crashing.
First, try to rebuild the mailboxes from Mail.app. Select the mailbox containing the bad note in the left navigation of Mail.app, and then select Mailbox > Rebuild, and see if the issue clears.
Failing that, log into your mail provider's web mail access — assuming your mail provider offers that service — and delete the note from there.
I tried the rebuild through iCloud mail but it doesn't appear to do anything. I also tried logging into iCloud remotely, but the note in question is not there. This note is also not on my iphone, and it's not listed on the main Notes page either. It only shows up on my Mac and I cannot get rid of it. It opens behind Notes when I open that app. I appreciate your help and will take any other suggestions you might have!