MonsterInsights filter that supports displaying the login name (or full name) rather than an internal ID in Top logged-in report.

mi-user-id-filter-functions.php

<?php

/**
 * Custom Filter for the User ID Dimension
 *
 * Add this to your theme's functions.php file.
 */
function display_username_instead_of_user_id( $user_id ) {
  $user = get_user_by( 'id', $user_id );
  return $user->user_login;
	// or return $user->first_name . ' ' . $user->last_name;
	// or return $user->user_login . ' ('. $user_id . ')';
}
add_filter( 'mi_get_user_by', 'display_username_instead_of_user_id', 10, 3 );

report-dimensions.php

<?php

/**
 * Apply a filter to display a more human-friendly string
 * rather than the internal WordPress user ID.
 *
 * /google-analytics-premium/pro/includes/admin/reports/report-dimensions.php
 */

// If logged-in user dimension, then apply a filter 
// to display user login, name, etc. ~mlc.
if ( intval( $key ) === 5 ) $title = apply_filters( 'mi_get_user_by', $title );

Where does the apply_filters call go?

Insert the if condition in the prepare_report_data() function around line 128 of the report-dimensions.php file.

The Top Logged-in Report (before)

Example Result (after)