Which are some planes shaped like an inverted wing?
Your intuitive feeling that the fuselage could be used to create additional lift is correct. Indeed, some aircraft are specifically shaped such that fuselage could provide substantial lift (and that's excluding any 'flying wing' design where fuselage is completely blended with the wing).

However, these are generally supersonic fighter aircraft. At supersonic speeds, even the 'normal' tubular fuselage will create significant lift, given a decent angle of attack. Moreover, the engine air intakes, if properly designed, will create even more lift.

In GA aircraft, on the other hand, the effect must be negligible, and structural considerations (yes, including high/low wing) are much more important.

The primary aerodynamic consideration for the fuselage is, generally, minimising its drag and reducing interference with other parts, primarily the wing. In this regard, by the way, C172 (or any high-wing aircraft) is more 'lift-friendly'. High-wing design has less negative wing/fuselage interference; in fact, it can be even positive. In simple terms, the part of the wing above the fuselage still creates lift, and probably more of it than the whole fuselage (although I have no numbers at hand).

I also must duly note that the principle of lift generation has little to do with the geometric length of the upper and lower surfaces. It is a popular misconception. The air indeed moves faster over the upper surface, but this is due to entirely different reasons. There are several good explanations of lift on this site.