The one thing to know about Ghost
Ghost sites are typography-first and usually restrained, so a widget that inherits the site font and uses the Outline theme will pass unnoticed. A loud card layout will not.
Where reviews belong in a Ghost site
An HTML card inside a post or page, or code injection in the site header for something that appears everywhere.
Full-width sections suit carousel and grid layouts; sidebars and narrow columns suit a badge. Floating badges can be restricted to particular pages and to desktop or mobile.
Reviews and search results
The widget outputs review schema markup automatically. Whether Google shows a star rating depends on what the reviews are attached to — worth reading before you expect stars, because for a business reviewing itself the answer is usually no.
What does help either way: reviews on the page are content, and a page that answers what customers actually say is a page people stay on.