Writing on a banana isn’t wrong. It’s just not normal.
Writing on banana skins is similar to doodling on a piece of paper. It’s just another way to be creative and to express yourself. It can also be a work of art, although a temporary piece with a very limited shelf life. That’s why it’s important to photograph your work soon after completion as it could be the only record you ever have.
There’s even a Facebook page dedicated to banana writing and displaying banana art:
As long as the banana is eaten afterwards – no harm done!