I’ve been using this for years and can’t understand why it isn’t available in Mail’s GUI. Here’s what to do:
Quit Mail.app
Open Terminal.app
Enter defaults write com.apple.mail PreferPlainText -bool TRUE and press enter (toggle it back by changing TRUE to FALSE)
Open Mail.app
View an HTML message and marvel at the readable plain text
The catch is that this only works for messages that actually contain a plain text alternative, which far from all HTML email does.