Lightroom's camera viewfinder is a video feed provided by iOS platform APIs that have some processing applied and may look different than unprocessed raw images the same platform provides Lighroom during image capture. The capture viewfinder is an approximation of what to expect but as you've found does not always match unprocessed RAW capture. After capturing raw images with LR's camera, processing is needed and will help improve image appearance--often just a matter of adjusting exposure. With Raw, you have a lot more image data to work with, and so this is often still preferable to capturing JPGs instead, which will match the capture viewfinder more often.
I recommend you experiment with applying an import preset like Color > Natural in LR Settings > Import > Raw Default, or simply tap Auto in Edit view for the Raw DNGs that LR captures.
Raw images captured by the LR camera usually are editable once a little image processing