Even if $CFG->enablenotes is set to false in moodle config or the user doesn't have the capability moodle/notes:manage, the "Add a note" button is shown in a participants page in mobile moodle. If the user clicks on this button, a rather ugly error is shown:

XMLRPC Error
"Notes are not allowed" (translated from german error text, don't know the exact english error text)

I think the button should be hidden when the user isn't allowed to add notes.