I don't think anyone would be surprised or mad if those skins weren't released until after the multiplayer was completely finished.
As someone else already pointed out, if you guys do work on single player missions afterward, you'll need to build them anyway. But they certainly aren't a priority now. I wouldn't even want you guys to build them now. It's just sort of a "Hey, that's kind of neat" feature. Definitely at the very bottom of the priority list.