Frumpy is a bloody ridiculous word to describe a mostly-casual brand such as fat face. Most of their outdoors, casual cotton stuff is excellent for the price.
I don't rate the more 'fashion' pieces at all, but they're hardly frumpy. But I will say they don't get the best out of their online models - gorgeous women of all sizes but terrible poses and weird angled photos. The jewellery and underwear is hugely overpriced.
However, I don't see how fatface could be described as frumpy... if you, say, compare a puff sleeved viscose dress from the likes of Whistles that looks like it fell from a broken curtain rail for nearly £200. Now that's a laugh! Frumpy, whether you love or loathe the word usually describes someone who is probably on a low income, unwell or otherwise disengaged in terms of clothing. Not really something to laugh at, nor compare to a fairly expensive high street store that mostly caters to outdoorsy or coastal types. Fatface is good at what it does best - so the fashiony stuff sucks, naturally.
Perhaps people are referring to the tired, directionless middle class: perhaps a reaf of looking like one's mother in the 80's. Well, we all do anyway now with the tentlike aline midi dresses and skirts 
So yeh, I'm not a fan of fatface but I rate their quality for the price and practicality. I'd rather purchase a fatface hoodie if I had to than a thin shitty misshapen sweaty betty for £££££.
And they remind me oh home, of nature, my memories of climbing trees and wading through fresh water. It's wholesome, I suppose. That's a more appropriate word.
Frumpy to me is Pep (?) at Poundland, Peacocks, Bon Marche. Let's not deny a lot of these connotations are not just about age but also our perceptions of a 'lower class'.