After that, she will not complain anymore, as long as the player character stays neutral. The player character will get a second warning when their Karma is even lower (-150). However, when opening a conversation with Cass after she has joined the player character and when they have substantially negative Karma (-100) she will complain about their behavior. Karma does not determine which companions can be recruited.
Unlike reputation, where one can only gain fame or infamy (with a single, scripted exception), Karma can be both gained and lost.
Karma joins reputation as a game mechanic, using a 2000-point linear scale with -1000 being the most Evil, 0 Neutral, and +1000 the most Good.