So what went wrong? In a word: repetition. The mothership has a couple of distinctive areas (like a research lab dedicated to mechanical merry-go-round horses), but most of the ship's interiors look the same -- and worse, look just how you'd imagine a spaceship to look, with lots of metallic surfaces and straight lines, and so Bethesda doesn't get any points for creativity.
Then there are the enemies -- all three of them. Despite the DLC being combat-oriented, with almost nothing in the way of meaningful dialogue, quests or decisions, you keep fighting the same enemies -- aliens, robotic drones, and odd creatures called abominations -- over and over again. Worse, the enemies all drop the same limited number of alien items, including a grand total of four new types of weapons, and so looting your kills and the numerous containers in the mothership isn't a lot of fun, either.
How bad does it get? No less than nine times during the campaign you have to destroy an alien generator, but these sequences all go the same, and you're never asked to do anything else. Bethesda couldn't have thought up even one more thing for you to do during the campaign? One different way to disable the ship? Apparently not.
Sadly, the DLC is even a little bit sloppy. There's a missing PIPBoy image. All aliens show up in the interface as enemies, but you actually lose karma for killing some of them. You meet some other abductees on board the mothership, but their scripted dialogue often triggers at the wrong times (like when a soldier named Elliott keeps claiming that aliens are flanking you -- well before and well after the ambush). And after finally fighting your way to the alien captain and killing him, his body inexplicably disappears soon after the battle, likely before you've even had a chance to loot his unique weapon. Yay.
The Mothership Zeta DLC pack has a couple of fun sequences (including one battle where you essentially get to play pinball with the aliens), but most of its six-hour playing time is a boring slog consisting of killing the same things over and over again, looting the same things over and over again, and seeing the same sights over and over again. Unless that sort of repetition excites you, Mothership Zeta is a DLC pack to avoid.