When will you go? You must be excited.
Yes , there is as much of an adherence to formal wear in the Mauretania Restaurant as there is everywhere else. On formal nights this means tuxedos for men and long dresses for women. The dress code is requested and followed in all public rooms all night (including bars, lounges etc.) and passengers from all dinning rooms will meet there.
Yet, formal wear is not required every night.
There are also informal and casual nights. The brochures will give you the exact definition but my experience goes as follows:
Informal: Dark suit or jacket with tie for men, cocktail dresses for women
Casual: At least slacks and dress shirts for men, a significant number with light coloured suit or jacket and tie, "sunday dress" for women.
Enjoy your QE2 experience!