Whenever I sign out of Bloodborne or play offline, I notice my messages ratings never seem to change. Does this mean other players can only see my messages if I am actively playing online?
Yes, your messages only appear to other players, who are within your level range, while you are playing online. This is probably for a couple reasons:
The notes you've made are tied to your character and they can vary from character to character. For instance, I have five character profiles and have written different notes with each and only the notes for that character are visible to other players when I'm playing as that character.
There is a limit to the number of notes you can have active at one time. Your newer notes will overwrite your older notes (unless you've "protected" the note from being deleted in this manner, then only unprotected ones will be overwritten with your newer notes). If all your notes were always loaded then you'd excede this limit.