Is the mothership zeta dlc worth buying

It depends on what determins worth to you. It comes with, in my opinion, the best energy weapons in the game, it adds a unique questline, it also provides nice loot worth a decent amount of caps, and it also provides nice health restoring items, and allows you to get the Alien Epoxy which can repair signficantly damaged weapons without actually needing the repair parts. Very useful for things like the Sniper Rifle which breaks easily and is near impossible to find. It also works on items added by the other DLC's that are harder to find in the main game (such as the repeating rifle from Point Lookout, and the various electroaxes from the Pitt. Theoretically it could also work for the Mes as well.