Mixed pitch is a minor mode that enables mixing fixed-pitch (also known as fixed-width or monospace) and variable-pitch (AKA "proportional") fonts. It tries to be smart about which fonts get which face. Fonts that look like code, org-tables, and such remain fixed-pitch and everything else becomes variable-pitch. The variable ~mixed-pitch-fixed-pitch-faces~ is a list of faces that will remain fixed-pitch in mixed-pitch-mode:
[[file:screenshots/screenshot_20170712_202445.png]]
(use-package mixed-pitch :hook ;; If you want it in all text modes: (text-mode . mixed-pitch-mode))