I think this category of clothes falls into a similar category of Granny Chic.
If you are young (ish) or thin (ish) or have expensive other things (haircut, shoes) then this stuff can look fab on you. It says: I don't care about fashion in an overt way, I have money though to spend on this lovely unstructured stuff.
If you are fatter, dumpy, don't have the expensive hair/shoes/jewellery, then like with Granny Chic, it just looks plain.
Young girls wearing Granny Chic look cool, hip, and they blend it in with sexier clothing. Middle-aged women wearing Granny Chic look like grannies, which is fine if that's the look you are going for, not if not.
These 'plain' clothes are exactly the same, if they are genuinely worn by plain people with no other accessories/wealth markers, they would look like sacks, but if they are part of an overall look, on the right person they can look charming/deliberately simple.