jpegmini works by compressing an image as much as possible while keeping the distortion calculated by there particular metric below a threshold. To compare against mozjpeg you'd need to compress a bunch of images with JPEGMini and then compress those images with mozjpeg so that they were the same size. You could then compare the quality of the resulting images.
The test set of images is here: https://github.com/bdaehlie/web_image_formats/tree/master/im...
If you're interested, https://github.com/dwbuiten/smallfry is program similar to JPEGMini that uses mozjpeg as its backend.