There is no definitive idea of frumpy; it clearly varies from person to person.
For me, if I'm feeling run down, or haven't had enough sun and fresh air, have eaten badly and my skin is pale, pasty and zitty, my clothes fitting a bit tightly, I lose confidence in myself and dress to hide myself. I sling on the scruffy jeans, dull t shirt, clumpy boots and drab coat. I have four kids between 5 and 10 years and so I have spent many years slobbing around in dull, scruffy clothes and doing the school run in clothes to make me invisible. I'm rubbish at making myself look polished in the morning and usually during the winter months I pull on a big woolly hat to hide myself even more.
When I feel well, I generally feel good about myself when I look in the mirror (healthy glowing skin, bright eyes etc) and so tend to choose my outfit more carefully. I've generally been quite a daring dresser throughout my life and enjoyed attention (punk, mod sort of style), so I ignore the rules and love experimenting with clothing. However, as I get older (I'm 37 now) I am a little saggier and my hair is thinner and greyer so I take much more care about what colours suit me. I have spent a while working out what my season is (to be honest I am still a bit unsure - I think I'm on the cusp) but I know that bright and high contrast outfits make me look washed out now, and cooler, dusky, low contrast colours accentuate my blue eyes and make me skin look good. That said, I also like to wear bizarre clashing combos - I think I can get away with it - and I enjoy playing dressing up at home to put pieces together that you would never imagine work.
I always put comfort high on my list of priorities - I have sensory processing disorder - and even if I love a stylish piece of clothing, if it feels uncomfy I can't wear it. I only recently discovered that a lot of my 'frumpy' clothes were actually a size too big for me. I bought a pair of shorts that fitted really beautifully and realised they were smaller than most of my jeans so when I got home I looked at myself in my jeans and could see how baggy they were on my hips and bum. I'd just not noticed as I was so used to them.
I don't agree that you need to spend a fortune to get good clothes that look good on you. I have a mixture of stuff that I've found at jumble sales, charity shops and more expensive high street brands. I don't own any expensive jewellery, but I do wear costume jewellery. I love nail polish. I always feel good when my hair style is good (it's a bit lank and boring right now) and this usually encourages me to play around with my clothing.
Anyway, more me 'frumpy' just means not caring about your appearance, although does it really matter, actually?