Trivial user.get api does not seem to support all the capabilities frontend has - specifically, "is online?" column is populated from the session data and in trunk r70896, there's a direct sql query in users.php:437 :
$dbSessions = DBselect(
'SELECT s.userid,MAX(s.lastaccess) AS lastaccess,s.status'.
' FROM sessions s'.
' WHERE '.dbConditionInt('s.userid', zbx_objectValues($data['users'], 'userid')).
' GROUP BY s.userid,s.status'
);
it would be useful to expose this in the api