When I updated to WordPress 7, I noticed the font in the post editor/publisher was kind of messed up. Turns out, it’s a part of the theme you’re using. If your theme doesn’t have specific things specified, you can get some ugly fallbacks.

The fix?

In any Block Editor page, such as editing a post, 3 dot menu, Preferences, then Appearance, and then disable “Use theme styles.”

https://wordpress.org/support/topic/editor-font-is-wrong/