It's not all moms. My mom didn't do it; I don't do it.
But moms *do* feel that we are judged by how our children dress. And we are.
(I always wanted to give my little kids a button to wear that said, 'I dressed myself'--first to motivate them to do it, and then to point out that THEY get the blame for wearing stripes with plaids, etc.)
As for how to respond, I like Oh Joy's suggestion.
I also suggest you be affectionately amused. She loves you; she feels tightly linked to you--those are good things. She's mentally stuck in her image of you--that's sort of funny. (Or, it can be if you LET it be, if you let go of it.)
As for degree of financial independence--once modesty and basic respect are met, it's not your mom's business. I feel that until my kids are about 20 (one is 18 right now), I have a right to say, "that's not appropriate attire for dinner at someone's house//church/a funeral--go put on a shirt with a collar/suit/something fancier."
And the only time I think money enters into it is this one: "Don't you dare wear your good shirt to go work on the car!"