I mean, she was okay doing this in the 2nd season of TD, and she and TJ married in the middle of the 1st season. Right now, that's the ultimate no no for her. She also wore low-cut dresses while pushing her boobs up until the 3rd season or so, which is another big no no for her now.
Plus she DOES show quite a lot on IG, about every Monday. She just doesn't pushes her boobs when she should while doing so or take pics with bad quality and/or lighting most of the time. Even in the pic above, if she pushed her boobs into the middle, they would look much better in the very same bikini. All of this suggests that she doesn't know rather doesn't want.
Even when she nails it perfectly like she did here (tanning her boobs and wearing the bikini right), it all changes when the cameras start rolling. When she wore the same bikini during TD filming, she wore it in a way that covered more and with no tan at all. The same applies to her revealing IG pics vs non-revealing gears. This may suggest that either she doesn't like to show off in front of recording cameras or that, as you said, she has confidence issues, which would be terrible shame since she has a fantastic body.