Add term of current custom post type to admin body class using admin_body_class

I think you’re missing get_current_screen().

add_filter( 'admin_body_class', 'rw_admin_body_class' );
function rw_admin_body_class( $classes ) {
    $screen = get_current_screen();
    if ( 'post' != $screen->base ) {
        return $classes;
    }

    global $post;
    $terms = wp_get_post_terms( $post->ID, 'product_cat', array( 'fields' => 'all' ) );
    $terms = wp_list_pluck( $terms, 'slug' );
    foreach ( $terms as $term )
    {
        $classes .= ' my_taxonomy-' . $term;
    }

    return $classes;
}