To be fair Hancock is not one dimensional at all. Far from it. Her attitude before and when she met Luffy was that of complete arrogance and distrust to mask her insecurities. It wasn't love at first sight. In fact Hancock hated men in general because of what was done to her. But it was only through allowing herself to get to know Luffy and putting aside her fear and prejudice that she fell in love with him. This allowed her to change her attitude towards her subordinates and other men into a more understanding one.
Allow me to blatantly copy/paste from Wikipedia without "tweaking"
"Because of the horrors she endured from the World Nobles, Hancock became a very bitter and cold woman, caring nothing for others (except her sisters) and usually maintains a tough and aloof demenor. She maintain this attitude due to her belief she should be strong for her people and out of fear of being taking advantage of again. After falling in love with Luffy, she's become a lot more sympathetic and kinder. She now smiles more often and has started to treat her fellow Kuja's with much more kindness and respect and has even decided to do work with them rather than just order them around as usual."
Instead of seeing the big heart eyes and the gags, try looking at her on a deeper level. The experiences and time it took for her to get to that point, where she could actually love a man that much, were tough. It didn't happen overnight. And while she's not that prominently featured as some side characters are, her development is clear to see to anyone who cares to look beneath the surface. And saying or implying she's one dimensional is not looking beneath the surface. Hancock as a woman on her own is admirable. Her love for Luffy only adds to that.