What a mess!
(Does it mean the following?)
What a mess it is!
What a mess the room is!
All three sentences are correct.
The "mess" is referring either to a terribly cluttered (and maybe dirty) room or house or something like that.
(Does it mean the following?)
What a mess it is!
What a mess the room is!
The "mess" is referring either to a terribly cluttered (and maybe dirty) room or house or something like that.