I suppose it depends on how observant we are generally. My hubby is not at all. He can walk through a room and not notice things that have fallen on the floor, and if I ask him to move the red box in the porch he will go in there, look, and come back to say he can't see a red box!
Years ago, after finally managing to lose most of the excess 3 stones I had acquired with my two pregnancies, we were going to a garden party. I bought a lovely floral dress that cost about twice what I would usually pay from C and A, and he said "I haven't seen that dress before, has Jane given it to you?". Jane was 5 stones heavier than me. The dress went back to the shop.
But like another poster, he doesn't notice his own clothes either. We have both deliberately lost weight recently (about 1.5 stones each) and he decided he could now fit into clothes he hadn't worn for a while. We were visiting our daughter's in-laws who are always smartly dressed, and I noticed that the trousers he had chosen were not only stained on the front, but frayed round the bottoms - clearly very well-worn.
It takes all sorts, but I do think a husband and wife should take note of their other-half's appearance at least to some extent. My hubby too tells me an outfit looks nice when I know it doesn't.