Are you asking whether you should advertise on Neopets or create a website similiar to Neopets? I went down that road years ago and it is by no means easy to setup. As for places to get traffic you can go join gaming topsites (topwebgames.com), advertise on kids websites and post in virtual pet forums like virtualpetlist.com to give you a jump start. If your website is original though, which trust me is hard to do, word of mouth should work the best.
Marketing gaming sites is easy. I can do it for less then .20 per signup and do it by the thousands. The problem is how do you get around neopets trademark?