That is now how reflections work. The incidence angle has to be the same as the reflected ray angle. It would make sense if Medaka could see the front upskirt if the puddle was directly in front of Mona. But there ain't NO WAY that he could see her butt, specially if the puddle was behind her.
God, I was so pissed off that I actually made a rough diagram of how it may work given that the puddle was on a slope.