How to disable jetpack/css/jetpack.css file ?

That file is a concatenated version of the CSS available in Jetpack. Indeed, instead of including a separate CSS file for each Jetpack feature, we’ve decided to include only one file, to limit the number of requests made when loading your site.

You can disable that option by adding the following to your theme’s functions.php file, or to a functionality plugin:

add_filter( 'jetpack_implode_frontend_css', '__return_false' );