Maybe the very things that made Spike unique, his sense of loyalty and his ability to feel love for another being and consider their wishes and welfare before his own, were the things that repulsed Darla. Certainly it was illustrated, time and again, that she felt none of those emotions. Even her so called affection for "her boy" seemed to me to be more possessiveness and pride in just how evil and despicable Angelus, as her offspring and protege, was. Certainly, once he got his soul and was no longer eager to rape, torture, and murder to her satisfaction, she not only lost interest in him, she actually threw him out.
In a strange way, Darla's disapproval of Spike could actually be seen as a positive character assessment.