If you made no other changes, then yes, encoding with a binary format like CBOR makes little difference and wouldn’t be worth it.
Where it makes sense is if you also allow native binary packets. That is, make the id, author, content and signature fields raw bytes. Keep tags as strings.
A common tag might be “mime-type”. Additional compression optional.
This would open the door for using Nostr to relay non-text data without paying the base64 or URL encoding cost to cram binary data into the content string. Additional data structures can themselves be nested CBOR binary content etc.