Hi! I am a 18 years old girl, who lifts weights three times a week. I go to bodypump classes once a week in addition to that. I’ve read that you build muscle, as long as you exhaust yourself. Bodypump is fun, but I won’t do it if it’s a waste of time 
My goal is hypertrophy and low body fat.
Well, chances are there are better ways you can use that time and energy if your main goal is hypertrophy and low body fat. I’m guessing you already know that because you posted the question. You’d be better served directing that energy into the three existing lifting sessions or adding a fourth “real” lifting session.
However, if it’s fun, and your homegirls like that more than regular lifting or you have friends that go to the class and really like it, and you can do with without taking too much out of you, perhaps it’s a good active-recovery kind of thing from your harder workouts.
Really depends how advanced you are and how serious your goals are. If you want to step on a stage someday, perfection might be necessary. If you just want to look good in a bikini this summer, the BodyPump classes might be an acceptable indulgence.