What I had in mind for frumpy-looking jeans are uncropped mom jeans with unfashionable shoes. Edit: pants that were perceived as frumpy/"mumsy" in the mid 2000s.
Some visual examples in the spoiler below:

These ones? https://vulrien-sims.tumblr.com/pos...nymous-user/amp

Or these?
https://e-neillan.tumblr.com/post/1...nim-jeans-4/amp

Or these?
https://deedee-sims.tumblr.com/post...s-m2f-these-are

There's these as defaults https://rudhira.tumblr.com/post/652...-these-ive-hado

Plus if you go through the default database you'll see there's a few that could work, they generally have links to the originals so you don't have to default. https://sims2defaults.dreamwidth.org/189630.html

@ekrubynaffit the ones by DeeDee and Rudhira are the best ones! Since I play a story set in the 00s, the ones with Adidas sneakers look too fashionable for mumsy Sims, though.

Who says that mums can't be fashionable? Sneakers would be comfy to wear while chasing after the kids.

@charity that's a good point! I forgot to mention in my OP that I meant pants + shoes that were perceived as frumpy back in the 2000s.

I'm a mum from the 2000s and I had Adidas sneakers while being poor as mud!


But heres some more that could work

https://e-neillan.tumblr.com/post/1...ans-af-2-colors

https://janika31.tumblr.com/post/66...it-was-a-little

https://gulioss-sims.tumblr.com/pos...-this-outfit-by

https://liketotallysims2.tumblr.com...f-only-18#notes

Other thing you could do is find the pants you like, find some shoes you like and take it to the GOS wishing tree and ask if someone there could put the shoes on the pants mesh.