Why do girls look so much better when they're in feminine clothing
They may look so much better to you and others however it doesn't mean they look so much better in general. Likely they look so much better to some people because it's often the expected thing as well as feminine tends to mean weak/submissive/subservient and many guys tend to want to be strong/dominant/controlling and seeing the weak often makes people feel strong or stronger.
Girls clothes are generally made it fit their shape, so the top of a shirt may be stretchier and enhances their shape, maybe causing them to look more attractive. The same goes for shorts and pants, the style is generally tight as short as possible, showing off and seeming more attractive.
Males are socialized to appreciate and be attracted to the female body and also be the pursuers. When women get all dolled up they look better because it is what males want to see. It fulfills our expectations.