how to create custom post type with dynamic category_name using shortcode

The arguments for your myCPT() function are incorrect. Callbacks used for add_shortcode() received two arguments, in this order:

1. An array that contains the parameters used with the shortcode.
2. The that is between the opening and closing shortcode, if supported.

You code is assigning the attributes to $query (and then overwriting them), and trying to get the attributes from the shortcode content.

So you code needs to be:

function myCPT( $atts, $content ) {
    $option = shortcode_atts( array( 'cat' => '' ), $atts ); // This needs to be the same as the first argument. 

    $query = new WP_query( array(
        'post_type'     => 'myposttype',
        'category_name' => $option['cat']
    ) );

    // etc.
}
add_shortcode( 'myshortcode', 'myCPT' );