Add extra content to RSS feed

The problem lies in these lines:

    $fv_jobtype = get_the_terms($post_id, "job_type");
    ...
    $output .= print_r($fv_jobtype);

First line takes all job types assigned to given post and stores it as an array of objects in fv_jobtype variable.

Second last one uses print_r function which tries to print given complex value in readable form – so you get what you get.

How to fix it?

Output what you really want to see there. If only name should be visible, do exactly that:

function crunchify_feed( $content ) {  
    if ( is_feed() ) {  
        $post_id = get_the_ID(); // sample reference. remove this if you don't want to use this
        $output = implode( ', ', wp_list_pluck( get_the_terms( $post_id, 'job_type' ), 'name' ) ); 
        $content = $content.$output;
    }  
    return $content;  
}

add_filter( 'the_content', 'crunchify_feed' );