Curious if the OP ever went to meeting and what his/her impression was.
Quakers are about as uninterested in how you dress as you can get. At the meeting I attend, one older woman usually wears a dress, while younger people are in T-shirts and jeans. One guy usually comes in his bike-riding gear, fresh off a ride to meeting.
Quakers put an emphasis on simplicity, and that can extend to simplicity of dress for many. In short, don't overthink it. If your clothes are clean and modest, you're good to go.
Interestingly, there are a few Quakers who still embrace the plain dress of days gone by. Not as extreme as the guy on the Quaker Oats box, but more like what you might expect the Amish to wear -- men in simple button-down shirts, with denim pants and suspenders, and women in solid-color ankle-length dresses, some wearing bonnets and some without. Quaker Jane's website has a lot of interesting thoughts on how she adopted plain dress -- although she doesn't do it anymore:
Quaker Jane - Limiting your palette in an addle-pated world. Plain dress, quakers and spirituality.
Again, though, it's an
extremely small minority of Quakers who observe plain dress. The vast majority just wear everyday modern clothes.