Depends on the person....some might take it offensive. Some might not.

It depends on the context. A man looking at a sexy woman might say that, but he means it in a positive way. He is being ironic when he calls it indecent, because he actually likes to see women wearing skimpy clothes that show a lot of her body

Yes, it's very offensive to speak to someone that way about her clothes, in a Western culture. It can also be prurient, particularly if used to talk to a third party about someone's clothes. prurient /ˈprʊərɪənt/ adjective: prurient having or encouraging an excessive interest in sexual matters, especially the sexual activity of others. "she'd been the subject of much prurient curiosity"

Hello Salvater! The word "decent" means of an appropriate, or generally acceptable nature or morally acceptable. So if the skirt was "indecent" it means that it was not morally acceptable and would be offensive by certain societal standards. With regards to your sentence: "That skirt of hers is positively indecent." I would say that it is a very polite way of saying that the skirt is unacceptable. It would be impolite if the person in the sentence was slandered- perhaps by calling her a cheap name because if the skirt, or describing the skirt with a "nasty" adjective. I hope that helps!

Here the definition of indecent for this sentence: "(of clothes) showing parts of the body that are usually covered" https://www.oxfordlearnersdictionaries.com/definition/english/indecent?q=indecent