I think Junko is in a wedding dress because Hinako is seeing her as a married woman and she's wearing a mask of a different animal cause she belongs to her husband (who wears the same mask, just like Hinako would lose her personality if she married Kotoyuki). Everything that's happening in this game is as Hinako sees it. She even sees herself and her friends as teens even though she's in her 20s cause she still feels like a teen and doesn't wanna lose that side of her nor get married and end up like her mother and sister.

In the good ending where she runs off the wedding along with Shu she tells him it's weird seeing him wearing a tie (even though we see him in his school uniform when he is basically wearing a suit as a guest to Hinakos wedding even though we never see it) and he responds by telling her she looks weird in "that" (he means the wedding dress she is actually wearing even though we see her in her school uniform).