I'm not sure t-shirts of that calibre are referencing Churchill. I think it's more likely to be a reflection on 'beer googles' making ugly people seem more attractive. A person who is ugly even when you are drunk must be very ugly indeed.
The original - though there seems to be many versions was: Bessie Braddock: Sir, you are drunk. Churchill: Madam, you are ugly. In the morning, I shall be sober. (Bessie Braddock was a very fat Liverpool Labour MP) Some people ascribe the quotation to Nancy Astor, but she wasn't ugly. One of her spats with Churchill was: Sir, if you were my husband, I would give you poison. Churchill: If I were your husband I would take it.