Do they mean the same thing? Does look up mean "lift the skirt and look" whereas look under would be going under a woman's skirt and looking up? Sorry, it's hard to know what the phrasal verbs exactly mean for a non-native speaker.
In idiomatic English, to "look up" a female person's skirt is to deliberately direct the gaze so that areas of the wearer's anatomy, underwear, etc, that are normally hidden by the skirt, can be seen. To do this is a serious breach of good manners and privacy, and may be deemed indecent behaviour or sexual harassment. An equally deplored act is to "look down" the neck opening of a female's shirt or dress, so that her chest area is seen. If such areas become visible by accident, e.g. if the female person is bending over, the appropriate action is to discreetly direct the gaze elsewhere.
The phrasal verbs "look up" and "look down" are to be interpreted as looking upwards or downwards into an aperture or opening. One can look up a chimney or look down a well or hole in the ground. "Look up" is not to be interpreted as consulting an index or catalogue. Generally, to "look under" something, e.g. a cloth or cover, implies lifting it first.