This has happened to me. In the older grades at my new school, the boys would look up my skirt. There was one person who was laying on the floor behind me when I was "squatting" to put my backpack down. I turned around and I saw him giving me a flirty look. I rolled my eyes and walked away. So yes, it is creepy and don't do it without expecting to be slapped.
Yes, looking up someone's skirt is extremely creepy, because a person has a right to their sense of person, even if they are walking on the street. A person has a right to walk down the street without someone looking up their skirt. You can look at their skirt, but you have to go out of your way to look up it, and that is super creepy.
Looking up skirts is a violation of privacy. It is no different than looking into someone's window when they are undressing or getting dressed. How would the person who is doing this like it if it was their wife, daughter or female relative. These people should be punished and given treatment.
Yes, looking up someone's skirt is creepy and wrong. It is an invasion of privacy and it is a form of sexual harassment. A person that looks up a woman's skirt should be faced with an arrest because it tells the person that it is a serious offence; other than a fine, which is seen as minor.
Whatever people do in private is their own business, looking up a persons skirt without their permission is a breach of a person fundamental right to personal space and privacy. It is a kind of sexual assault, nothing more and nothing less, and should always be treated as so. Allowing anyone to get away with it would be a slippery slope.