Hi Josh,
I successfully installed Pothos on Ubuntu 16.04 (but I upgraded to the 4.6.3 kernel for reasons independent of Pothos). When activated the topology, I obtained the expected message about needing to run /usr/lib/x86_64-linux-gnu/uhd/utils/uhd_images_downloader.py in order to obtain the image for the uhd (b200 mini) from ettus.
For some reason, there is no utils directory. After googling, I discovered that the .py files was in /usr/lib/uhd/utils/uhd_images_downloader.py. Running that as sudo successfully downloaded and installed the image.
I don't know how often this happens, but it might merit an update to the message.
Tony