smart-listing-post-classes.php

<?php
function my_smart_listing_classes( $classes ) {

	global $post;
	$terms = get_the_terms( $post->ID, 'category' );

	if ( ! empty( $terms ) ) {
		foreach ( $terms as $term ) {
			$classes[] = 'category-' . $term->slug;
		}
	}

	return $classes;
}

add_filter( 'jet-blog/smart-listing/post/classes', 'my_smart_listing_classes' );