You know what this question reminds me of, it's when Hagrid takes Harry to the Diagon Alley to shop, and he's just flabbergasted by seeing magic, even though he knew that the magic exists but seeing it performed by everyone on the street is what shocked him . The point is if you don't know something or haven't tried it before, it doesn't mean it doesn't exist. Yes, you can absolutely wear a costume from a web series; Halloween is about having the looks of your favorite character right! Even if you like the Karate Kid renewed series, yes, you can easily get behind the Scarlet witch coat
without making it look weird.