The case sensitivity was my issue too! I think I pulled the github down *Wunderground* it installed with the folder name in lowercase, but the files themselves *WunderGround* and I was using MMM-Wunderground in my config.
Once I symlinked the correct capitalization, and corrected my config, it works perfectly.

20 years using linux, one would think I would catch this myself!

Thanks for asking the right questions! This thread got me up and running.