The aspect of the dashboard camera stream can be changed when editing a camera from a dashboard. You can edit the dashboard/camera by selecting the gear icon at the top right when viewing a dashboard or long pressing the dashboard from the dashboards list.
There are three aspect options available, Fit, Stretch, and Zoom. Fit will letterbox the image to ensure the stream is displayed fully without any distortion. Stretch will stretch/distort the image so the full stream is shown within the bounds of the view. Zoom will crop the image so the stream is shown in the correct aspect ratio and remove any extra off the top or bottom of the image to fit the bounds.