I love them!
Frumpy is a state of mind imo, it's about not feeling relaxed in yourself and/ or not finding joy in your clothes. There's absolutely nothing wrong with frumpy of course! There are obviously more pressing concerns for most humans.
Ironically I find people who obsess too much about not being frumpy veer unwittingly towards being frumpy. Too fussy, not relaxed, dressing to conform rather than for personality or joy.
I have a friend who is a dancer. Dresses in clothes for comfort and movement, not a brand label in sight. Maybe her outfit alone would be described as frumpy on here. But she's not frumpy, her physical confidence shines through her everyday movement, so the clothes don't look frumpy on her.