Help implementing snippet

Regarding the snippet posted here, meant to display the rating on homepage listings, and considering I’m a very poor excuse for a newbie coder, I must ask for help implementing it.

My original intention was to place the rating over the featured image, on the lower right corner. I think it would look awesome, but I can’t achieve that, too dum :/ (I’m using the Customizr theme, if anyone can give me a pointer, I’d appreciate it).

That being impossible, I thought I’d make a shortcode to show the rating anywhere I’d like inside the content, and if that worked, I’d go for a better approach (one step at a time: first a shortcode, then an action hook, and then failure xD).
This is what my shortcode looks like:

add_shortcode('rating', 'wppr_rating');
function wppr_rating(){
    $output = '';
    // get the ID of the post as $post_id
    $review = new WPPR_Review_Model(get_the_ID());
    $rating = $review->get_rating();
    // display the rating icon or use $rating to display a custom icon
    $output .= 'placeholder text ' . wppr_layout_get_rating( $review, 'donut', 'default', array( 'review-wu-grade' ) );
    return $output;
}

Good news, the shortcode works; bad news, the rating donut shows up over the top border of the rating block, and not where I placed it. In fact, no matter where I place it, it always shows there.
What am I doing wrong…?