sku-display.js

jQuery(function ($) {
    var sku_fields = '[name^="variable_sku"]',
        display_class = 'variation_sku_display',
        warning_display = true;
    $('body').on('input', sku_fields, function () {
        var $input = $(this), sku = this.value,
            $variation = $input.closest('.woocommerce_variation'),
            $display = $('.' + display_class, $variation);

        if (!$display.length) {
            $display = $('<span />', {class: display_class})
                .css({
                    fontWeight: 'normal'//,
                    //float: 'right'
                })
                .insertAfter($('.remove_variation', $variation));
        }

        if (sku.length) {
            $display.html('<strong>SKU:</strong> ' + sku + ' — ');
        } else {
            $display.hide();
        }

        if (!$display.parent().length && warning_display) {
            alert('The plugin woocommerce-SKUs is not compatible with this woocommerce version. The tab structure for product variations probably changed. Please contact Dragon Media Group.');
        }

    }).find(sku_fields).trigger('input');
});

woocommerce-skus.php

<?php

/*
Plugin Name: Woocommerce SKU display

Description: Shows the SKU next to a variation.
Author: Ruben Vincenten | Dragon Media Group B.V.
Version: 1.0
Author URI: http://dragonmediagroup.nl
*/

// directory should contain this file + a js file.

add_action( 'woocommerce_product_data_panels', function () {
	if(is_admin()) {
		wp_enqueue_script( 'woocommerce-skus', plugin_dir_url( __FILE__ ) . '/sku-display.js', array(
			'jquery',
			'wc-admin-variation-meta-boxes'
		), 1.0, true );
	}
} );