I have found that since "informal" is so ambiguous for the women that no one can really define what is "proper". If you are neat and clean and wearing a slacks/sweater combo no one would look aghast. I have seen everything on formal nights from beautiful ball gowns to Sunday-go-to-meeting dresses. I think if the attempt is made no one can fault you.
If you have done your research and understand the type of cruise that Celebrity is trying to present, then you are aware of the dress code. But to book the cruise and then complain about having to dress up is childish. Everyone wants it "their way". I think that since the alternative dining is offered, then take advantage of it or dine in your cabin on formal night. But you can at least try to clean up if you want to go to the theatre or some of the other entertainment in the evening. This is a simple courtesy to yourself as well as your fellow passengers.