How to say "obscene": translate 下流 or 輕佻?
'Obscene' is not 下流(no class/ bad taste) or 輕佻 (frivolous)

One definition of obscene is 不堪入目 , it is used to describe perverted, immoral act or behavior 

For example: Taking a child from his mother and claim it is all for enforcing the law (which is not true) is an obscene (不堪入耳- too perverted and immoral to the ears) argument

Another definition of obscene is 淫褻 or 不雅 (also means 'unsightly'), it is used to describe extreme sexually suggestive content or display 

I would describe the skirt 不雅, 不堪入目 or 淫褻 if I was judgmental, but I rather call it 大膽 (bold) or 開放 (open)

暴露 (revealing) is also a fitting description of that skirt 

Edit: from all the answers: "下流, 輕佻, 不堪入目, 大膽, 開放, 淫褻, 變態, 不三不四, 有伤风化" are all 'subjective opinions' (you can't say any of them is wrong, but you can disagree with all of them)

Only 暴露 is  an 'objective description' (it is a correct statement based on fact. You can't argue with fact, that skirt does expose a lot of skin)