This is more like extremism than any religion, or any culture really. And comparing a fashion faux pas to extremist ideals is a bit facetious. Really.
Look, any religion taken to the extreme has some awful, awful, archaic moralistic rules. Why? Because they were founded centuries ago and much of their legal aspects were meant for that era and that era's people. Exhibit A - Leviticus and the Crusades. Exhibit B - Hudud punishments in the Qu'ran. And for that era, keeping populations isolated, controlled and fearful about outsiders was very much about preserving culture and life. Heck, you see that going on in today's politics - no immigrants, no refugees, protect "our" country's culture etc. Keeping the population in check has always been how powerful people stayed powerful and creating fear of outsiders (us vs them mentality) kept populations cohesive and easier to control.
As someone of Chinese heritage, if the cheongsam is done tastefully, respectfully...more Nicole Kidman than Chun Li, it's fine. Don't mock culture, and if someone says you're mocking it, then accept that you are. But yeah, let the minorities decide where the line is...the whole tumblr/twitter moralistic crowd who try to tell us where the line is are also part of the problem. But also don't deny that cultural appropriation happens and that minorities have a right to speak up about them. And yes, if a culture accepts aspects of loaning their traditional attire out for tourism and cultural education, it's still THEIR decision within a specific and limited circumstance, it's not a blanket "okay" for brands to run off and send modified kimonos, cheongsams, hanboks down the runways and onto red carpets.
There is a middle ground though and listening is it.
Nick is no Sikh, but he and his fashion minions have suffered no indignant uproar. He has stated he doesn't even believe in religion. Nick Cannon: “I’ve been studying the Sikh faith. It didn’t start as a fashion statement but it kind of turned into one.” All rightey then, you do you, Nick 
