Replace comments with a dummy text

Yes, possible.

Add this to your theme’s functions.php file-

add_filter( 'get_comment_text', 'replace_comment_texts' );

function replace_comment_texts() {
    return "This comment is hidden";
}

I have written a plugin few days back, that does the exact thing. You can try this too- https://wordpress.org/plugins/mask-comments/