When JPGs are captured on your phone, Apple's APIs are used and the JPGs get processing that make them look the way they do. When Lightroom Mobile DNG images are captured on your newer iPhone, the newer sensor provides a lot more image data and these images are not processed heavily like JPGs are. The camera viewfinder may look different from the final output because it's actually a video feed (constantly updating) but still gets lots of system processing to look the way it does. We're exploring solutions. As a workaround in the meantime, if you find your DNG captures are coming out overexposed, try using Exposure Compensation setting -- drag your finger left or right over the camera viewfinder to find the best setting (from -3 to 0 to +3). When decreasing exposure a little (example: -1), the camera viewfinder may look darker than the actual DNG produced. If you end up underexposing your image, this is still easier to work with in the editor than an overexposed image.
Thanks for taking the time to post your feedback. It informs us how important it is that we keep working on improvements for this.
--Charlie