@Ihopeyourcakeisshit
I know that there are one or two members of MN who seem to bask in the glory of being the poster girls for 'telling it like it is' but a wee bit of diplomacy is not always a bad thing.
Do you really believe that about diplomacy Ihopeyourcakeisshit? So if you were looking to buy a dress on say something like Facebook Marketplace, so had seen a photo of the dress but couldn't try it on before buying it, and "it was on at such a good bargain price" that you wanted to get it for a special event, but you weren't quite sure for several reasons, would you be happy that the friends you asked for advice from were diplomatic, or would you prefer them to be honest?
Diplomacy has it's place when you meet your friend at a party and think that the dress she is wearing is both hideous and innapropriate for the event, because it would be too late to do anything about it then, but when someone asks you for advice before they are committed to an action, and you lie, then you are the horrible person, not the person who is trying to give the helpful advice they asked for.
HelpMeDressForXmas, I'm sorry but the dress does look very cheap; the thinness and light weightness of it, together with the cheap and gaudy sequins, and the brassiness of the "gold" colour, are what makes it look cheap in my opinion, also its cut doesn't help either. As for it being appropriate for a Black Tie event, no it isn't. Maybe you could do a little research on tinternet to get some examples of appropriate wear?
However, having said all that OP, I doubt very much that you would be denied entry to the Christmas meal/party, and what you wear won't change you from being a kind and thoughtful person, to a horrible and selfish one. It also won't make the slightest difference to how well you do your job. Any person at your Christmas "do" who makes their distaste for whatever you are wearing obvious to you, and/or the other people there, is the one who deserves judging badly. So, of course it is up to you OP, but I wouldn't want one of my daughters wearing it, as I think that they would be judged by some people, and if they realised it then not only could it ruin that night, but also make them feel uncomfortable back at the work place.