As a real person who uses an email client (might not be real) who does see the text/plain parts, my feeling is Discourse is above average here. I have a little nit that the text is text/markdown, not text/plain, but it looks a lot better than some sites that obviously just use very poor html to text filters. “Oh, look, it forgot to strip the style sheet!”
The “minor nit” bit from the first post is probably accurate.