Going back for the girl, for any human being is motivated enough at any time your totally right.
Going back for a cat is bordering on the insane. What if she actually already were portrayed as a mother in Alien? Going back for a cat with her daughter waiting at home? I dont even know how i would call such a person.
Put yourself in Camerons shoes. Now you wanna make Ripley a mother for whatever reasons. You actually cant because of her going back for the cat. No sane mother would put her life and her responsibility for her child on the line for an animal. That would shift her from a somehow crazy but likeable character to an irresponsible, reckless mother immediately.
And her actions in Alien diminish her going back for Newt in Aliens even if she wasnt/isnt a mother. A crazy character can not act heroic, his actions will always be determined by this one thing: his crazyness. Today hes saving a little girl, tomorrow a canary bird, the next day he doesnt bother at all; its totally random. You can like the crazy caracter for saving the child, but you can not admire him for his actual motivation (overcoming his fears, caring for other people etc.).
Im not actually saying Ripley is potrayed as being crazy in Alien, yet for me going back for the cat is a crazy kamikazesque action. You can overlook it, sure hollywood (Awww shes going - for the cat), but it might actually pull you out.