What is the aesthetic standard of calligraphy? Why are some calligraphy works called "untidy" and "untidy"?

The beauty of calligraphy does not mean that neatness is beauty. There are many styles of Chinese characters, with different fonts, running scripts and cursive scripts in different times, and some of them are very beautiful.

Your question also puts forward a standard of beauty, and what is beauty is a bigger problem.

Good cursive works look messy but neat, not graffiti. .....