I also finished the 3-night Dream cruise on a Monday. I did notice the note "optional dress up" on the front corner of Sunday's Navigator, but it was easy to miss. I don't know if our servers mentioned it the night before because I was at Remy on Saturday. Most people didn't seem much more dressed up on Sunday than on Friday. There were photos being taken in the atrium on Sunday, as usual on the "dressier" nights of our longer cruises; I am not sure whether there were also photographers on the other nights.
If you want to wear a fancy dress, go for it -- there will be others dressed up, even if many aren't.