No mobile viewport
Effort to fix: Quick win (under an hour)
Why it matters
Without a viewport tag, phones render the desktop layout zoomed out. Most visitors are on mobile, and Google indexes the mobile version.
How to fix it
Add <meta name="viewport" content="width=device-width, initial-scale=1"> to the <head>, then check the layout is responsive.
On WordPress
Modern themes include it; if yours doesn't, switch to a responsive theme or add the tag via the theme's header.php (child theme).