Would it be inappropriate to go to a fancy dress party dressed as an ultra-orthodox jew?
It might not be inappropriate to *be* at the party dressed as an ultra-Orthodox Jew (the men in your link are Satmar Hassidim), depending on how you act. However I think it would be inappropriate to *go* to the party dressed that way. Someone who sees you in passing on the street probably won’t understand that you’re supposed to be in fancy-dress. Your behavior may disrespect or discredit Orthodox Jews in ways you wouldn’t even be aware of. Or, by dressing-up to mimic them, you might well offend a passer-by who doesn’t grasp why you’ve done it. It’s really a questionable thing to do! If you must do this, I suggest you not put on key elements of your costume until you arrive at the party. Or wear something else.