The fashionistas think that anything that isn't in fashion is frumpy, and are reluctant to state what they wear when challenged.
The rest think that anything that is dowdy, drab coloured, shapeless, badly fitting and basically is unflattering on the wearer is frumpy.
"Frumpy" is used on here usually about anything which isn't "flattering" or "doesn't show off your figure".
By "fashionistas" if you mean the posters who are probably going to be on the Parisian thread or who will suggest non mainstream brands, I really don't think that is the group who call anything"frumpy". I never do, but lordy, I've posted choices and I've had my choices ripped to shreds by some posters as being "frumpy" because they don't meet the above criteria.
I can't respond to your second paragraph. My clothes might not be "flattering" by the standards set by some posters but they are never dowdy, drab coloured, shapeless or badly fitting.