Does a herbivore gain 1 star if he attacks but the damage is ignored due to armor plating?
Yes, Alieniod with Herbivore should gain 1 Star for not damaging anyone.

Herbivore specifically tells you to gain a Star if you don't damage anyone, so assuming Alienoid doesn't buy a card that damages anyone, they will gain 1 Star from Herbivore. The rules specifically call out the matter:


  If an attack somehow ends up doing no damage (due to Camouflage for example), then it is no longer considered an attack for the purposes of cards like Poison Spit or Herbivore.