I never took philosophy like Karen Moy or Harlan Jones, but as a layman let me see if I can understand how Nietzsche's comment is relevant here. Clearly, this statement feels cynical. Fear is not a positive emotion, therefore, morality in this sense is the negative result of this negative emotion. When is morality negative? The word 'morality' is only negative when it is merely the appearance of goodness, rather than coming from a sense of love and duty for the welfare of our fellow beings. Can someone be truly moral when they are simply trying to appear as such for the approval of others? So in this context, the quote chosen is in reference to Tommy's desire to appear moral while he is actually keeping secrets about his past from Brandy, which is not moral or ethical.
Now that I have thought about it, I have to give Karen Moy chops for this quote... which is really disappointing because I wanted to make fun of her for it. Because there is simply nothing else in today's entire strip to make fun of!!!