Do your conditional ternary before you format the time object moment creates.
var sunset_time = global.get('homeassistant').homeAssistant.states["sun.sun"].attributes.next_setting;
var current_time = moment().local();
var sunset = moment.utc(sunset_time).local();
var test = moment(current_time).isBefore(moment(sunset))? "Yes" : "No"
var msg = {"payload": "current: "+current_time.format('HH:mm')+" - sunset: "+sunset.format('HH:mm'), "before sunset? ": test};
return msg;
or something similar, untested.