Hmm I'll take a look and see if there are any unneccessary npcs in the underworld/moon maps. Granted this list'll be short at first: it's pretty hot over here and I can't be on here long sadly.
1) Zeromus Fallen is used twice in the lunar core, only because one of them is set to march. Not sure how important that one is, but I think it could be nixed (or the non marching one). That would mean tweaking that event (final battle or ending) so the other npcs can move properly on that one map.
2)Hummingways - There are SEVEN of these useless guys crowding the Hummingway map on the moon. I'm sure you could take two or even three of those away and no one would call fowl. All they do is hum, and even if you were to take three of them out, there'd still be nine basically identical guys walking around, with four of them still giving the useless humming message lol. This would probably be the so-easy-it's-rediculous route. :)
Alright, my computer room's beginning to feel unbearable, which is sad given the short amount of time I'v been on here. Hope that helps! I'll try to hop back on later to do a little more searching if needed.
We have EVEN MORE themes now, but some old ones are gone...