I find this interesting actually and it’s daft to pretend it doesn’t exist - I know I can look frumpy or mumsy or dowdy or matronly or whatever word you want to use, if I dress a certain way. I’m always keen to work out why. I’m not very thin, I’m short and have big boobs but if I dress to flatter my figure rather than try to hide it I don’t scrub up too bad.
I think it’s definitely not just about weight (although that can contribute) For me personally no nos would include:
Very plain clothes with few accessories. But too many can also look wrong, as though they are wearing the person rather than the other way round. I’ve ditched scarves for this reason - I love them but they feel far too fussy now
Costume jewellery (statement necklaces etc) but also very traditional gold chain, watch, rings etc. Chunky silver or something a bit arty looks much cooler
Very practical clothes - sensible dress, flat shoes, functional bag etc
Outdoorsy stuff like anoraks or those awful trousers/sandals especially if worn with smarter things eg jeans, leather boots, sensible bag.
Some floral patterns, tunics etc (White Stuff are very bad for this especially those made out of dishrag type material)
Shoes picked for comfort not style
There are a few colours that I don’t think ever look good especially some of those colour experts recommend - anything too bright and flat or sickly pastels
Clothes that don’t fit, especially underwear - sadly the matronly bosom is a thing
Not even attempting to keep up with fashion, wearing the same look and hairstyle and makeup for years
Generally dressing for comfort or practicality rather than to ‘make the best of yourself’ which might be sexist and ageist and misogynistic but basically most of us want to look nice
I think it’s a combination of looking like you’ve made no effort or don’t care about clothes and a coming across as a bit try-hard - a very fine balance and one I certainly don’t always get right!