I think large scale automated harmonic analysis is a worthy endeavor for the purposes of musicological research that could even be applied to pedagogy (deceptively hard problem: identify what piece(s) of music to teach to achieve specific goals for students).

But you really need good (and preferably ethical) sources of data to do that, and UltimateGuitar ain't it. You also probably want to engage with some music theorists to normalize the data to give you better analysis and ask better questions than "what is the most common chord."

From this analysis I don't think "is music getting simpler" can be answered, and I think the trends are interesting questions to investigate for musicologists but this data set and analysis are too flawed to answer them.