Can a platoon sgt tell a female soldier that she is required to wear a bra when in civilians?

Unless regs have changed the answer is no. An exception is what is worn on a base whose commander sets the code.

I think you should any way

No. As long as you’re off duty. NCOs don’t make policy; they enforce them. Your commander is the leader to ask, not RallyPoint.

Sounds like a EO complaint eating to happen.

As a female who served in the 80s - take it up w/the platoon sergeant & then up the chain. I think for her to tell a male platoon leader to tell a female this, is outside the bounds. (If it were in uniform, maybe - in civilian attire, not even close.)
Now: I never wore a bra in the field because the pack wore sores on my shoulder. I rarely wore 1 off-duty, even in a coed barracks - bras were not fashionable or comfortable. That said - in Tunisia, I wore one all the time. In Greece, only in uniform. (H-e-double hockey sticks: I even went topless on the beach! And my command knew it.)
SO: soldier, you need to find out WHY she thought you should wear one. If it's because of local custom (or because your boobs are a distraction), you need to wear one.

As former military I would say it’s situational discretion. Granted your platoon Sargent could’ve come directly to you as discussed this issue in further detail. Or have the your team lead tell to see her privately. And sadly to say she might put you team lead in trouble if taken the wrong way. Can of worms if will.

From what I’ve read you team lead is respected by you and I didn’t know how nor he did about the situation. I would’ve had you go see the platoon sarge with me.
And try to get more clarity of her issue.

Given the possible places you can be stationed in I would take that as factor of going bra less. Mid East for experience and example you should be wear one regardless of being gifted or not.

Send a picture so we can judge if it was inappropriate!

The military owns you, I got wrote up for getting a sunburn

Unless she has a significant (distracting)bounce,let her enjoy her time off.

Besides the aforementioned (and well thought out) earlier points, here's a couple of thoughts that sprang to my mind that I haven't seen in the thread... (Better late than never I suppose)

1. Are we certain that the PltSgt actually issued the request? As a guy, I'd consider it quite fortunate if I could "direct" my underlings appearance and have a convenient place to shift blame/better the optics of said request (I "ain't a stick in the mud" for telling you to conform to my belief that ladies need to wear appropriate upper undergarments, I'll claim another lady recommended you do so...)

2. Have you considered "aggressive compliance"? Whichever style of "barely there" garment you prefer/can tolerate/allows you your comfort that I'm fairly sure exists somewhere that indicate your adherence to at least the letter of the request (as long as said act of semi-defiance doesn't run afoul of the other reasons given earlier like local custom etc.)

3. Most likely, if your shirt/blouse is "snug enough" to clearly reveal that you aren't wearing a bra, then it's "snug enough" to allow indications of... other issues... that affect the female anatomy there...