She might want to try the Gap Everyday Smooth bra:
https://www.gap.com/browse/category.do?cid=34524&style=1114195
I think I have similar complaints about most bras as your DD, and these are the best option I've found assuming that what she's really looking for is smoothing under clothes and not compression. For compression, she pretty much has to stick with sports bras...that's what they are designed for and you need that much coverage to achieve compression. There are some sports bras with thinner straps, but they usually have racerbacks so don't work with a lot of clothes.
Separately, you might gently try to talk to your DD about being comfortable with having breasts (since it sounds like she might be self-conscious about them). No one ever did that for me, and it's led to a lot of body issues over my life.