Why not just select a note on the map and open the Text Window, from the contextual menu or ⌥⌘X?
A problem with the suggestion is that text bodies can be very long, contain images, links, and other features not obvious on the body of the note on the map. I think if you could “directly edit” text, then many people would have a bad experience.
(Text Window has a nice feature that if you open it from several notes, then Text Window adds a new tab for each note. Cool.)