How to check homepage contain blog post?

In the case of expound theme index page is used as homepage, and there is no static front-page in the theme.

Try to add is_home() to the condition.

function addtoexcerpt($excerpt) {
    ob_start();
    $values = ob_get_clean();
    return $values.$excerpt;
}
if( is_single()||is_category()||is_home() ) {
    add_filter('the_excerpt','addtoexcerpt');
}

I hope this should help you.