This is a bit of a "dirty" joke.
Speaker A is "aiming" at the man's "stomach" in order to reach or win his "heart."
Speaker B is suggesting that instead of stomach, Speaker A should target an organ located below the stomach (and in fact, located below the waistline and between the legs...).
In regular conversation, to "aim a bit (too) high" is to have excessively high or unrealistic aspirations or ambitions. So, the phrase is used with a double meaning in this case.
Additionally, one could say that a third meaning is involved: "high" can be used to refer to something nobler or spiritual or intellectual, while "low" can be used to refer to baser, darker, more "natural" instincts or behaviors.