UPDATED: AUGUST 6TH 2017
We always see girls out in fancy dress, guys as well. Your safest bet would be to head for Mathew Street which is renown for the fancy dress and hen party scene here in Liverpool. Lots of cool bars on Mathew Street who will gladly accommodate parties in fancy dress, including: Flares, Boogie Nights and the Rubber soul with many other great venues nearby.
You can also head up to Concert Square which has a massive selection of clubs and bars to choose from. Some trendy venues will probably turn you away but most bars, pubs and good clubs will let you in.
Concert square does attract the younger parties, so if you're part of a mature crowd you might want to stay around the Mathew Street area, though saying that, it's all just a good laugh regardless where you go out in Liverpool.
In short, you'll be fine so many places about Liverpool to party in fancy dress, just stay away from the super trendy clubs and you’ll be ok.