I think frumpy is when clothing is designed purely with function in mind and no thought is given to how it will look on a person's real life body. So for me, it's things like knee length shorts, fleece jackets. Perfectly functional but not at all about appearance.
Sometimes the designer will try to put some weird detail to make it look more interesting, but it's badly thought out and a bit random - so those black plain ankle boots with a wrinkle effect across the top or when M&S puts frills and buttons and shit on otherwise plain but okay clothes. Those details that don't add anything that makes an item look better.
Mostly clothes are made for slim, well proportioned women - it is much harder to dress a big bust, especially on a budget because clothes are made as cheaply as possible now and aren't cut for women with more shape.
I suppose I think of frumpy dressing as wearing something ill fitting just because it covers you, even if the trousers are saggy or the hems too short and not caring about this.
And I think it might be in wearing a formulaic look, so that you don't have to think about your own individual style - I think the patterned dress/leather or denim jacket/trainers look has spilled into this now. It's a way of being invisible almost. And that's what I think frumpy might be - trying to not be noticed or stand out in any way.
For men it's jeans/trainers/t shirt that don't have any thought put into the design - just bland cover up clothes.
FWIW I don't think expensive branded sportswear is better than lots of unbranded equivalents - it all just looks better on fit, toned, slim bodies, regardless of where it's from.
And I love skinnies and ankle boots.
But to avoid 'frumpy' I think you do have to consider body shape, colouring, the items that you personally feel happy in and a little bit of what is fashionable.