I would like to keep a password matrix in an Evernote note. I would like to ask for feedback on the security of that storage.
No. Evernote's own description of their encryption indicates that they use 64-bit RC4. 64-bits is way too short, and RC4 is generally considered broken.
Encryption aside, I've never gotten the impression that Evernote was design around high security, so I'd avoid using it for highly sensitive data like passwords. Consider tools designed for storing passwords like Keepass or LastPass. They may still have weaknesses, but at least their engineers are designing with password storage in mind.