If you've ever studied Palmistry, you would notice that each mount on your hand has a different symbol.
For example, the mount of Jupiter on your hand, which is your index finger (pointer finger) is a symbol of luck and fortune. The base of your ring finger is the mount of the Sun, which is a symbol of the arts and culture. Your middle finger, which is also the finger people flip others off with, is the mount of Saturn; a symbol of misfortune.
Hotaru had a lot of misfortune in her life. In the manga, she was killed as a child and made into a cyborg by her estranged father, in the anime she was almost killed. Later, when Tomoe made a deal with Master Pharoah Ninety and Mistress Nine, Hotaru was completely weakened and possessed. She has no friends at school, she stays at home in her fathers dark and dingy mansion and she's generally sick and weak.
If this isn't bad luck, what is?
Furthermore, in certain aspects such as stars, grids and points on your hand, Saturn can be a symbol of power. Correct me if I'm wrong, but Sailorsaturn isn't exactly the pinnacle of weakness...