Hi Hugo,

By default, you can leave a review for the main variable product only. So, you also can show the review rating of the main product for all variations.

Please add the below custom code (How to add custom code?)

add_filter( 'woocommerce_product_get_rating_html', 'woocommerce_variation_get_rating_html', 99 );
function woocommerce_variation_get_rating_html( $rating_html ) {
    global $product;
    if ( $product && $product->is_type( 'variation' ) ) {
        $parent = wc_get_product( $product->get_parent_id() );
        $rating = $parent->get_average_rating();
        $count  = 0;
        if ( 0 < $rating ) {
            $label = sprintf( __( 'Rated %s out of 5', 'woocommerce' ), $rating );
            return '<div class="star-rating" role="img" aria-label="' . esc_attr( $label ) . '">' . wc_get_star_rating_html( $rating, $count ) . '</div>';
        }
    }
    return $rating_html;
}