It depends. Is she modest and doesn't want to display her parts? Good. Has she been body-shamed and told she's fat, or is she seeing current definitions of "beauty" like size 0 models and perfect skin? Is she picking up some of the flak about breast feeding moms being so "disgusting" as to show their breasts for their intended purpose, and she's applied that to herself? Bad.
Does she have breast buds and she thinks they shouldn't show? Let her wear a shirt or camisole-type garment under her shirt. Has anyone said anything to her, or touched her? Has anyone talked about her, or someone else, as having a fat belly or being "slutty" or something else along those lines? Find out.
If she's planning to wear leggings all summer, she's going to be good and hot. But if she wants a one piece bathing suit or something with shorts instead of a bikini bottom, fine. She'll be more comfortable than the girls whose suits are riding up so their cheeks hang out or losing their tops in the waves.
Keep an eye on her - the only thing that sticks out to be is feeling "sad". Otherwise, let it go, let her find her own style, and see how it progresses. Sometimes the more we worry, the more we transmit some anxiety to our kids unconsciously.