I think it's a polite way of requesting that emails not use garish fonts and colors. Just imagine how many submissions he must get in illegible "handwriting" fonts.
Parent uses "Mutt", an email program using a UI so old it's probably endorsed by the Society For Creative Anachronism; the notion of "fonts" or "colors" in email, however garish, is incomprehensible to him. For that email client, content is ASCII text, nothing more.