The Neki have a non-standard sitting pose arms position, as it seems to (subjectively) fit their overall "theme" and appearance a bit better than the vanilla one does.
Unfortunately, due to the way the frames are implemented in Starbound, this causes the sleeves of most armors/clothes to appear detached from the arms while sitting, with no way to configure it per-species currently present in the game.

What it should look like[i.ibb.co]
What it ends up looking like[i.ibb.co]

There are a few workarounds to choose from:

• Install NekiSit, to fix it, but appy the Neki sitting pose to all species
  
• Install NekiNoSit, to apply the vanilla sitting pose to the Neki
  
• Install BetterSitIdle together with the Neki Addon, to apply an alternate sitting pose to the Neki and all other species
  
• Use Invisible cosmetic armor/clothes