Deactivate mycred-widget-css?

Sure.

Add this to your themes functions.php file:

add_filter( 'mycred_remove_widget_css', 'remove_mycred_widget_css' );
function remove_mycred_widget_css() { return true; }

Alternatively you can always de-register any script / style that has been registered with WordPress via the wp_deregister_style or wp_deregister_script functions. You just have to make sure you run these functions after the original script was registered by the plugin. Otherwise you will de-register something that does not yet exist.