Get wp_title wp ajax

YOAST’s SEO Title is stored within a meta_key.
Try this-

$taxonomy = get_queried_object()->taxonomy;
$term_id = get_queried_object()->term_id;
$meta   = get_option( 'wpseo_taxonomy_meta' );
$title  = $meta[$taxonomy][$term_id]['wpseo_title'];

Or this-

$titles = get_option( 'wpseo_titles' );