The accessibility team has concerns about the implementation of this functionality by default, as we expressed previously in ticket https://core.trac.wordpress.org/ticket/12423.

• Our main concern is that Codemirror is not accessible for screen reader users.
• There should be an option to enable/disable Codemirror by the user, to fall back to the current (fully accessible) plain text mode.
• In the case of the theme and plugin editor and the CSS editor for the customiser, it can be an opt-out, because these aren't essential parts of using WordPress for a regular (non coding) user.
• If Codemirror is going to be implemented in for example the text editor for posts or the HTML widget, it has to be an opt-in. The text editor there is an essential part of WP's functionality.

On a personal note: I've installed the plugin and it's really good work, as a coder I like it very much.
My personal points of view are at: https://core.trac.wordpress.org/ticket/12423#comment:133