Ahhh... unless I'm missing something, the leftmost room in the above screenshot is missing a door. That will cost you a lot of points right there... it's important for rooms to be fully enclosed.
They don't have to be of any particular shape, and can even be bizarre, twisty caves for all the dwarves care, but they do have to be totally enclosed or you get a penalty.
Remember that you can decorate a room's walls twice. That room doesn't even have an entirely decorated floor...
But the easiest way to raise a room's value is to put high-quality items in it, especially masterwork ones. Have whoever your legendary dwarf is create some things, then throw them in there, even if they aren't needed.
Even if you lack a legendary dwarf, a mildly well-made item (with symbols around its name, like -this-) will still help. Even ones with no special quality will add their value and help bring the room higher.
Of course, having anything in the room that the room's occupant likes helps even more... or ore in the walls that they like, that sort of thing.
Did you remember to expand the room's radius to totally fill the area you enclosed it in? That big room is actually hurting you if you didn't expand your room's area to encompass it, since then your dwarf sees their 'room' as being an absurd chalk outline ala "I Love Lucy" in a much larger space, and will treat it as being missing walls.